Student Resource
Subject B-6a: Aircraft Materials and Corrosion
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
CONTENTS Definition
4
Study Resources
5
Introduction
7
Aircraft Materials – Ferrous
B-6a 1 - 1
Aircraft Materials - Non-Ferrous
B-6a 2 - 1
Aircraft Materials – Composite / Non-Metallic
B-6a 3 - 1
Corrosion Fundamentals and Identification
B-6a 4 - 1
Aircraft Rivets
B-6a 5.4 - 1
Pipes and Unions
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DEFINITIONS Define
To describe the nature or basic qualities of.
To state the precise meaning of (a word or sense of a word).
State
Specify in words or writing.
To set forth in words; declare.
Identify
To establish the identity of.
Itemise.
List Describe
Represent in words enabling hearer or reader to form an idea of an object or process.
To tell the facts, details, or particulars of something verbally or in writing.
Explain
Make known in detail.
Offer reason for cause and effect.
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STUDY RESOURCES B-6a Student Resource Jeppesen General Jeppesen Airframe AC 43.13-1B/ AC 43.13-2A Combined – Aircraft Inspection and Repair Aviation Maintenance Technician – Airframe by Dale Crane Aviation Maintenance Technician – General by Dale Crane Glencoe Aviation Technology Series – Aircraft Maintenance and Repair Advanced Composites by Cindy Foreman Metalurgy Fundamentals By Daniel A Brandt Fundamentals of Aircraft Material Factors by Charles E Dole Aviation Maintenance Technician Handbook (2008) FAA
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INTRODUCTION The purpose of this subject is to familiarise you with aircraft ferrous, non-ferrous and nonmetalic (composite) materials and their characteristics, properties, treatment, defects and relationship to the electro chemical series and corrosion. On completion of the following topics you will be able to: Topic 6.1.1 Aircraft Materials – Ferrous
Identify common alloy steels used in aircraft and describe their characteristics and properties. Describe heat treatment methods and application of alloy steels used in aircraft. Topic 6.1.2 Aircraft Materials – Ferrous
Define testing of ferrous materials for:
Hardness
Tensile strength
Fatigue strength
Impact resistance
Topic 6.2.1 Aircraft Materials – Nonferrous. Identify common nonferrous materials used in aircraft and describe their characteristics and properties. Describe heat treatment methods and application of nonferrous materials used in aircraft. Topic 6.2.2 Aircraft Materials – Nonferrous. Define testing of nonferrous materials for:
Hardness
Tensile strength
Fatigue strength
Impact resistance
Topic 6.3.1.1 Aircraft Materials – Composite / Non-Metallic Identify common composite and non-metallic materials, other than wood, used in aircraft and describe their characteristics and properties. List the sealing and bonding agents utilised with composite and non-metallic materials and describe their use. Describe special requirements for the handling and storage of common composite and non metallic materials. Topic 6.3.1.2 Aircraft Materials – Composite/Non –Metallic (Defects) Describe methods used to detect defects in composite materials and common repair methods.
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Topic 6.4.1 Corrosion (Fundamentals) Identify the relationship of various aircraft structural materials to the ‘ElectroChemical Series’. Describe corrosion formation by the following:
Galvanic action process
Microbiological
Stress
Topic 6.4.2 Corrosion (Identification) Identify types of corrosion and describe their causes. Explain the susceptibility of different material types to corrosion. Topic 6.5.4 Aircraft Rivets Describe types of solid and blind rivets by:
Head marking,
Physical characteristics
Identification number
Describe the heat treatment process for selected rivets. Topic 6.6.1 Rigid and Flexible Pipes Identify and describe types of rigid and flexible pipes and their connectors used in aircraft. Topic 6.6.2 Standard Unions Identify and describe types of standard unions used in the following aircraft systems:
Hydraulic.
Fuel.
Oil.
Pneumatic
Air
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TOPIC 6.1.1 AND 6.1.2: AIRCRAFT MATERIALS - FERROUS PROPERTIES OF METALS A given metal can possess several properties. Among these are strength, hardness, malleability, ductility, brittleness, conductivity, expansion, elasticity, toughness, fusibility, and density.
Strength One way to classify metals is according to the amount of strength they possess. A metal’s strength is determined by the percentage of parent metal and other elements used to make an alloy. There are many different types of strength, including:
Tensile strength
Compressive strength
Shear strength
Torsional strength
Flexural strength
Fatigue strength
Impact strength
Each type of strength is a measure of how a metal reacts to a specific type of loading. Tensile Strength Tensile Strength is the ability for a piece of sheet metal to withstand stress in tension. There are three definitions in tensile strength:
Yield strength - the stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently
Ultimate strength - the maximum stress a material can withstand when subjected to tension, compression or shearing. It is the maximum stress on the stress-strain curve
Breaking strength - the stress coordinate on the stress-strain curve at the point of rupture
Stress vs. Strain curve for structural steel is shown as an example. Training Material Only Issue C: August 2009
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Reference numbers are: 1 - Ultimate Strength 2 - Yield Strength (elastic limit) 3 - Rupture 4 - Strain hardening region 5 - Necking region
Other strength
Compressive strength is the ability of a metal to withstand “pressing” or “squeezing together”.
Shear Strength is a metal’s ability to withstand shear stress.
Torsional strength is the ability to resist rotational shear.
Flexural strength is bending strength of a metal.
Fatigue strength, or Endurance strength, refers to the ability of a metal to resist repeated loading.
Impact strength measures the ability of a metal to resist shock.
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Hardness A metal’s hardness refers to its ability to resist cutting, penetration, or abrasion. The tensile strength of steel relates directly to its hardness, but for most metals this relationship is not absolute. Some metals are hardened through heat-treating or work hardening, while others are softened by a process called annealing.
Malleability A material’s ability to be bent, formed, or shaped without cracking or breaking is called malleability. Hardness and malleability are generally considered opposite characteristics. To help increase malleability, several metals are annealed, or softened. In this condition complex shapes can be formed. After forming is complete, the metal is then heat treated to increase its strength. A metal may be fully annealed when the forming is started, but hammering and shaping can harden it to such an extent that it must be re-annealed before forming is completed.
Ductility The ability of metal to be drawn into wire stock, extrusions, or rods is called ductility. Ductile metals are preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are often used for cowlings, fuselage and wing skins, and formed or extruded parts such as ribs, spars, and bulkheads.
Brittleness Brittleness describes a material’s tendency to break or shatter when exposed to stress, and is the opposite of ductility and malleability. A brittle metal is more apt to break or crack before it changes shape. Because structural metals are often subjected to shock loads, brittleness is not a desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals.
Conductivity Conductivity is the property which enables a metal to carry heat or electricity. If a metal is able to transmit heat it is said to be thermally conductive. However, before a metal can carry heat away from its source, it must first absorb it. This ability to conduct heat away is called heat exchange. The fins on the cylinder heads of an air cooled piston engine remove heat in this fashion. Metals that can carry heat also carry electrons, making them good electrical conductors. Electrical conductivity is the measure of a material’s ability to allow electron flow. A metal conductor can be a wire, an aircraft frame, or an engine. Because of their molecular structures, the best electrical conductors are gold, silver, copper, and aluminum.
Thermal Expansion The property of a metal to expand when heated and shrink when cooled is called thermal expansion. The amount of expansion or contraction is predictable at specific temperatures and is called its coefficient of expansion. All aircraft experience thermal expansion and contraction as the ambient temperature changes.
Elasticity Elasticity describes a metal’s tendency to return to its original shape after normal stretching and bending. The flexibility of spring steel used for the construction of landing gear is a good example of elasticity. Another form of elasticity is demonstrated when aircraft skins expand and contract when an aircraft is pressurized. A metal’s elastic limit is the point beyond which the metal does not return to its original shape after a deforming force is removed. Soft materials such as lead, copper, and pure aluminum have very low elastic limits, while the elastic limit of hard spring steel is very high. Training Material Only Issue C: August 2009
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Toughness Toughness is a material’s ability to resist tearing or breaking when it is bent or stretched. Hammer faces and wrenches are examples of metal that must be tough as well as hard to be useful.
Fusibility The ability of metal to be joined by heating and melting is defined as fusibility. To fuse metal means to melt two or more compatible pieces of metal into one continuous part. The correct term is called fusion joining or welding.
Density Density is a material’s mass per unit volume, and throughout this section the term is used to compare the weights of various metals.
FERROUS METAL Any alloy containing iron as its chief constituent is called a ferrous metal. The most common ferrous metal in aircraft structures is steel, an alloy of iron with a controlled amount of carbon added.
Iron Iron is a chemical element which is fairly soft, malleable, and ductile in its pure form. It is silvery white in colour and is quite heavy, having a density of 7.0 grams per cubic centimetre. Iron combines readily with oxygen to form iron oxide, which is more commonly known as rust. This is one reason why iron is usually mixed with various forms of carbon and other alloying agents or impurities.
Iron poured from a furnace into moulds is known as cast iron and normally contains more than two percent carbon and some silicon. Cast iron has few aircraft applications because of its low strength-to-weight ratio. However, it is used in engines for items such as valve guides where its porosity and wear characteristics allow it to hold a lubricant film. It is also used in piston rings.
Steel To make steel, pig iron is re-melted in a special furnace. Pure oxygen is then forced through the molten metal where it combines with carbon and burns. A controlled amount of carbon is then put back into the molten metal along with other elements to produce the desired characteristics; the molten steel is then poured into moulds where it solidifies into ingots. The ingots are placed in a soaking pit where they are heated to a uniform temperature of about Training Material Only Issue C: August 2009
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1200oC (2,200°F). They are then taken from the soaking pit and passed through steel rollers to form plate or sheet steel. Much of the steel used in aircraft construction is made in electric furnaces, which allow better control of alloying agents than gas-fired furnaces. An electric furnace is loaded with scrap steel, lime stone, and flux. Carbon electrodes are lowered into the steel, producing electric arcs between the steel and the carbon. The intense heat from the arcs melts the steel and the impurities mix with the flux. Once the impurities are removed, controlled quantities of alloying agents are added, and the liquid metal is poured into moulds.
Composition of Steel Steel is a material composed primarily of iron. Most steel contains more than 90% iron. Many types of carbon steel contain more than 99% iron. All types of steel contain a second element—carbon. Many other alloying elements are used m most steel, but iron and carbon are the only elements found in all steel. The relationship of steel to cast iron and wrought iron is shown in Figure. 7
The difference in the three materials is primarily based on the carbon content. The percentage of carbon in steel ranges from just above 0% to approximately 2%. Most steel has between 0.15% and 1.0% carbon. Wrought iron contains essentially no carbon. Most types of cast iron contain 2% to 4% carbon. At approximately 6% carbon, the material becomes so brittle that it is relatively useless. Training Material Only Issue C: August 2009
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STEEL NUMBERING SYSTEM In general, the Society of Automotive Engineers (SAE) uses a four digit numerical index system to represent chemical composition standards for steel specifications:
The first digit identifies the principal alloying element
The second digit indicates the percentage of the principal alloying element
The last two digits indicate the average carbon content in hundredths of a percent
SAE STEEL NUMBERING SYSTEM The numbers assigned in the combined listing of standard steels issued by the SAE (Society of Automotive Engineers) and AISI (American Iron and Steel Institute) represent the type of steel and makes it possible to readily identify the principal elements in the material. SAE designation for major classifications of steel:
1xxx - Carbon steels
2xxx - Nickel steels
3xxx - Nickel-chromium steels
4xxx - Molybdenum steels
5xxx - Chromium steels
6xxx - Chromium-vanadium steels
7xxx - Tungsten steels
8xxx - Nickel-chromium-molybdenum steels
9xxx - Silicon-manganese steels
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Some are identified by a 3 digit AISI (American Iron and Steel Institute) system and others are designated by the manufacturer.
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ALLOYING AGENTS IN STEEL Iron has few practical uses in its pure state. However, adding small amounts of other materials to molten iron dramatically changes its properties. Some of the more common alloying agents include carbon, sulphur, silicon, phosphorous, nickel, and chromium.
Carbon Carbon is the most common alloying element found in steel. When mixed with iron, compounds of iron carbides called cementite form. It is the carbon in steel that allows the steel to be heat-treated to obtain varying degrees of hardness, strength, and toughness. The greater the carbon content, the more receptive steel is to heat treatment and, therefore, the higher its tensile strength and hardness. However, higher carbon content decreases the malleability and weldability of steel. Ferrous materials are generally classified according to their carbon content as shown in the table.
Wrought iron is not found in aircraft structures. Low carbon – Mild Steel in aircraft is primarily used in non structural areas but in the past, was used in steel tube fuselage construction. In sheet form these steels are used for secondary structures where loads are low. Low-carbon steel is easily welded and machines readily, but does not accept heat treatment well. Medium carbon steels will accept heat treatment. This steel is especially adaptable for machining or forging and where surface hardness is desirable. High carbon steels are very hard and are primarily used in springs, files, and some cutting tools. Cast iron has few aircraft applications because of its low strength-to-weight ratio. However it can be found in engine valve guides where its porosity and wear characteristics allow it to hold a lubricant film.
Sulphur Sulphur causes steel to be brittle when rolled or forged and, therefore, it must be removed in the refining process. If all the sulphur cannot be removed its effects can be countered by adding manganese. The manganese combines with the sulphur to form manganese sulphide, which does not harm the finished steel. In addition to eliminating sulphur and other oxides from steel, manganese improves a metal’s forging characteristics by making it less brittle at rolling and forging temperatures.
Silicon When silicon is alloyed with steel it acts as a hardener. When used in small quantities, it also improves ductility. Training Material Only Issue C: August 2009
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Phosphorous Phosphorous raises the yield strength of steel and improves low carbon steel’s resistance to atmospheric corrosion. However, no more than 0.05 per cent phosphorous is normally used in steel, since higher amounts cause the alloy to become brittle when cold.
Nickel Nickel adds strength and hardness to steel and increases its yield strength. It also slows the rate of hardening when steel is heat-treated, which increases the depth of hardening and produces a finer grain structure. The finer grain structure reduces steel’s tendency to warp and scale when heat-treated. SAE 2330 steel contains 3% nickel and 0.30% carbon, and is used in producing aircraft hardware such as bolts, nuts, rod ends, and pins.
Chromium Chromium is alloyed with steel to increase strength and hardness as well as improve its wear and corrosion resistance. Because of its characteristics, chromium steel is used in balls and rollers of antifriction bearings. In addition to its use as an alloying element in steel, chromium is electrolytically deposited on cylinder walls and bearing journals to provide a hard, wear-resistant surface.
Nickel-Chromium Steel Nickel toughens steel, and chromium hardens it. Therefore, when both elements are alloyed they give steel desirable characteristics for use in high-strength structural applications. Nickel-chrome steels such as SAE 3130, 3250 and 3435 are used for forged and machined parts requiring high strength, ductility, shock resistance and toughness.
Stainless Steel Stainless steel is a classification of corrosion-resistant steels that contain large amounts of chromium and nickel. Their strength and resistance to corrosion make them well suited for high-temperature applications such as firewalls and exhaust system components. These steels can be divided into three general groups based on their chemical structure:
Austenitic
Ferritic
Martensitic
Austenitic Austenitic stainless steels also referred to as 200 and 300 series stainless steels. A structure known as austenite forms when these steels are heated to a temperature above their critical range and held there. Austenite is a solid solution of pearlite, an alloy of iron and carbon, and gamma iron, which is a nonmagnetic form of iron. Austenitic stainless steels can be hardened only by cold working while heat treatment only serves to anneal them. They are non-magnetic in the annealed condition, although some may be slightly magnetic after cold-working. Ferritic stainless steels Ferritic Stainless Steels, which are part of the 400 series of stainless alloys, have chromium as their major alloying element and are typically low in carbon content. Ductility and formability are less than that of the austenitic grades. The corrosion resistance is comparable to that of the austenitic grades in certain applications. Thermal conductivity is Training Material Only Issue C: August 2009
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about half that of carbon steels. Ferritic stainless steels are magnetic; they generally have good ductility and can be welded or fabricated without difficulty. These grades can be hardened by cold rolling, but cannot be hardened as much as the austenitic alloys.
Martensitic stainless steels The major alloying addition in Martensitic stainless steels is chromium in the range of 11 to 17%. The carbon levels can vary from 0.10 to 0.65% in these alloys. This radically changes the behaviour of the Martensitic alloys relative to the Ferritic 400 Series alloys. The high carbon enables the material to be hardened by heating to a high temperature, followed by rapid cooling (quenching). Martensitic types offer a good combination of corrosion resistance and superior mechanical properties, as produced by heat treatment to develop maximum hardness, strength and resistance to abrasion and erosion. The Martensitic grades are usually sold in the soft state. This allows the customers to cut or form the parts before they are thermally hardened. They are magnetic.
Molybdenum One of the most widely used alloying elements for aircraft structural steel is molybdenum. It reduces the grain size of steel and increases both its impact strength and elastic limit. Molybdenum steels are extremely wear resistant and possess a great deal of fatigue strength. This accounts for its use in high-strength structural members and engine cylinder barrels. Chrome-molybdenum (chrome-moly) steel is the most commonly used alloy in aircraft. Its SAE designation of 4130 denotes an alloy of approximately 1 percent molybdenum and 0.30 percent carbon. It machines readily, is easily welded by either gas or electric arc, and responds well to heat treatment. Heat-treated SAE 4130 steel has an ultimate tensile strength about four times that of SAE 1025 steel, making it an ideal choice for landing gear structures and engine mounts. Furthermore, chrome-moly’s toughness and wear resistance make it a good material for engine cylinders and other highly stressed engine parts.
Vanadium When combined with chromium, vanadium produces a strong, tough, ductile steel alloy. Amounts up to 0.20 percent improve grain structure and increase both ultimate tensile strength and toughness. Most wrenches and ball bearings are made of chrome-vanadium steel.
Tungsten Tungsten has an extremely high melting point and adds this characteristic to steel it is alloyed with. Because tungsten steels retain their hardness at elevated operating temperatures, they are typically used for breaker contacts in magnetos and for high-speed cutting tools. Training Material Only Issue C: August 2009
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HEAT TREATMENT OF STEEL As mentioned before, pure iron is not suitable for use as a structural material. It is weak, soft, is very ductile and does not respond to heat treatment to any appreciable degree. Steel, which is basically iron alloyed with carbon and a few percent to a few tens of percent of other alloying elements can be heat treated to a wide range of strengths, toughnesses and ductilities. Carbon is the most important of these alloying elements in terms of the mechanical properties of steel and most heat treatments of steel are based primarily on controlling the distribution of carbon. Heat treating of steel is the process of heating and cooling of carbon steel to change the steel's physical and mechanical properties without changing the original shape and size. Heat treating is often associated with increasing the strength of the steel, but it can also be used to alter certain manufacturability objectives such as improve machinability, formability, restore ductility etc. Thus heat treating is a very useful process to help other manufacturing processes and also improve product performance by increasing strength or provides other desirable characteristics. High carbon steels are particularly suitable for heat treatment, since carbon steel respond well to heat treatment and the commercial use of steels exceeds that of any other material. There are many difference types of heat treating processes, it individual process provides different desirable characteristics to the product.
Iron-Carbon Equilibrium Diagram Iron is an allotropic metal, meaning it can exist in more than one type of lattice structure, depending on temperature. A stable Iron-carbon equilibrium diagram is showing in the figure.
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Pure molten iron begins to solidify at 2,800° F (1538°C). Its structure at this point is known as the Delta (δ) form. However, if cooled to 2,554°F (1401°C), the atoms rearrange themselves into the Gamma (γ) form, it is now called Austenite. Strangely enough, iron in this form is nonmagnetic. When nonmagnetic gamma iron in this form is cooled to 1,666°F (908 °C), another change occurs and the iron is transformed into a nonmagnetic form, called Alpha structure (α). As cooling continues to 1,414°F (768 °C), the material becomes magnetic with no further changes in its lattice structure. Austenite, Ferrite and Martensite are the different crystal structures that occur in steels as a result of heat treatment:
Austenite (gamma-iron) is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above its upper critical temperature
Ferrite (alpha-iron) is iron, or a solid solution with iron as the main constituent. In pure iron, ferrite is stable below its lower critical temperature
Martensite has a very similar crystalline structure to austenite and results from the rapid cooling of austenite during quenching
Face Centred Cubic (FCC) crystal – Austenite. This is the same ferrous metal heated above its upper critical temperature. This crystal structure is harder than a body centred cubic crystal.
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Body Centred Cubic (BCC) crystal – Ferrite. This is the crystal structure of un-heat treated ferrous metals below its lower critical temperature.
Body Centred Tetragonal (BCT) crystal - Martensite This is the same ferrous metal heated above its upper critical temperature and then rapidly quenched. This crystal structure is stronger, harder and more brittle than a body centred cubic crystal.
Critical Temperature The temperature at which a phase change occurs in a metal during heating or cooling. Above this temperature the steel crystal structure changes from Body Centred Cubic to Face Centred Cubic.
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Heat Treatment Process Annealing Annealing softens steel and relieves internal stress. To anneal steel, it is heated to about 50°F above its critical temperature, soaked for a specified time, and then cooled. The soaking time is typically around one hour per inch of material thickness. The steel can be cooled by leaving it in the furnace and allowing both the furnace and steel to cool together or by packing the steel in hot sand or ash so the heat is conducted away slowly.
Normalizing The processes of forging, welding, or machining usually leave stresses within steel that could lead to failure. These stresses are relieved in ferrous metals by a process known as normalizing. To normalize steel, it is heated to about 100°F above its upper critical temperature and held there until the metal is uniformly heat soaked. The steel is then removed from the furnace and allowed to cool in still air. Although this process does allow particles of carbon to precipitate out, the particles are not as large as those formed when steel is annealed. One of the most important uses of normalizing in aircraft work is on welded parts. When a part is welded, internal stresses and strains set up in the adjacent material. In addition, the weld itself is a cast structure whereas the surrounding material is wrought. These two types of structures have different grain sizes and, therefore, are not very compatible. To refine the grain structure as well as relieve the internal stresses, all welded parts should be normalized after fabrication.
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Hardening Pure iron, wrought iron, and extremely low-carbon steels cannot be hardened by heat treatment since they contain no hardening element. Cast iron, on the other hand, can be hardened, but the amount and type of heat treatment used is limited. For example, when cast iron is cooled rapidly, it forms white iron, which is hard and brittle. However, when cooled slowly, grey iron forms, which is soft but brittle under impact. Carbon steel can be hardened readily. The maximum hardness obtained by carbon steel depends almost entirely on the amount of carbon content. To harden steel, it is heated above its critical temperature so carbon can disperse uniformly in the iron matrix. Once this occurs, the alloy is cooled rapidly by quenching it in water, oil, or brine. The speed of the quench is determined by the quenching medium. Oil provides the slowest quench, and Brine is the most rapid.
Tempering Tempering reduces the undesirable qualities of Martensitic steel (brittle). To temper an alloy, it is heated to a level considerably below its critical temperature and held there until it becomes heat soaked. It is then allowed to cool to room temperature in still air. Tempering not only reduces hardness and brittleness, but also relieves stress and improves steel’s ductility and toughness.
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Determining Steel Temperature When steel must be heat treated without the aid of a pyrometer, its temperature can be estimated fairly accurately through the use of commercial crayons, pellets, or paints that melt at specific temperatures. The least accurate method of estimating temperature is by observing the colour of the material being heated. Some of the reasons why colour observation is inaccurate include the fact that the observed colour is affected by the amount of artificial and natural light and the ambient air temperature. However, as a last resort, when annealing or hardening non- structural components, colour observations can be used.
Case Hardening Certain components in aircraft engines and landing gear systems require metal with hard, durable bearing surfaces and core material that remains tough. This is accomplished through a process called case hardening. The steels best suited for case-hardening are the lowcarbon and low-alloy steels. If high-carbon steel is case-hardened, the hardness penetrates the core and causes brittleness.
The two methods presently used to case harden steel are:
Carburizing
Nitriding.
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Carburizing Three methods of this form of case hardening are:
Pack Carburising is achieved by enclosing the metal in a fire-clay container and packing it with carbon-rich material such as charcoal. The container is then sealed and heated to a temperature of 1,700°F (900°C). As the charcoal heats up, carbon monoxide gas forms and combines with the gamma iron in the metal’s surface. The depth to which the carbon penetrates depends upon the soaking time.
Gas carburising is similar to pack carburising, except the carbon monoxide is produced by a gas rather than a solid material.
Liquid carburising produces a high-carbon surface when heated in a molten bath of sodium cyanide or barium cyanide. Either of the components supplies the carbon needed to harden the metal’s surface.
Nitriding Nitriding is achieved by first hardening and tempering, then grinding the part to its finished dimensions. It is then heated to approximately 1000°F (540°C) and surrounded with Ammonia gas. The high temperature breaks the ammonia down into nitrogen and hydrogen. The nitrogen is absorbed into the steel as iron nitride. Most steels can be nitrided; however, special alloys are required for best results. These special alloys contain aluminium as one of the alloying elements and are called “nitralloys”. The depth of a nitrided surface depends on the length of time it is exposed to the ammonia gas. Aircraft engine crankshafts and cylinder walls are commonly nitrided for increased wear resistance. However, since nitrided surfaces are highly susceptible to pitting corrosion they must be protected from the air with a coating of oil.
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METAL TESTING Once the heat treatment process has been carried out, the material must then be tested to insure that the desired qualities have been achieved. Some of the properties that would be tested for are:
Hardness
Tensile strength
Fatigue strength
Impact resistance
Note – testing is carried out with a test piece consisting of the same material and thickness of component, which was heat treated with the component.
Hardness Testing Since the strength of most metals varies with hardness, it is often required to measure the hardness of a metal. The two most widely used methods of hardness measurement are the Brinell and Rockwell methods. The diagram below is a sample data producing by hardness testing.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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Brinell hardness tester The Brinell hardness tester uses a hydraulic force to impress a spherical penetrator into the surface of a sample. The amount of force used is approximately 3,000 kilograms for steel, and 500 kilograms for nonferrous metals. This force is hydraulically applied by a hand pump and read on a pressure gauge. When the sample is removed from the tester, the diameter of the impression is measured with a special calibrated microscope. The diameter of the impression is then converted into a Brinell number by using a chart furnished with the tester.
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Rockwell hardness tester The Rockwell hardness tester gives the same information the Brinell tester gives, except that it measures the depth to which the penetrator sinks into the material rather than the diameter of the impression. To use a Rockwell hardness tester, a sample is thoroughly cleaned, the two opposite surfaces are ground flat and parallel, and all scratches are polished out. The sample is then placed on the anvil of the tester and raised up against the penetrator. A 10 kilogram load, called the minor load, is applied and the machine is zeroed. A major load is then applied and the dial on the tester indicates the depth the penetrator sinks into the metal. Instead of indicating the depth of penetration in thousandths of an inch, it indicates in Rockwell numbers on either a red or a black scale. Rockwell testers use three types of penetrators: a conical diamond, a 1/16 inch ball, and a 1/8 inch ball. There are also three major loads: 60 kilograms, 100 kilograms, and 150 kilograms. The two most commonly used Rockwell scales are the B-scale for soft metals, which uses a 1/16 ball penetrator and a 100 kg major load, and the C-scale for hard metals, which uses the conical diamond penetrator and a 150 kg major load.
Procedure for Rockwell Hardness Test: The indenter moves down into position on the part surface A minor load (F0) is applied and a zero reference position is established Training Material Only Issue C: August 2009
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Major load (F1) is applied for a specified time period (dwell time) beyond zero The major load is released leaving the minor load applied The resulting Rockwell number represents the difference in depth (E - e) from the zero reference position as a result of the application of the major load
Tensile Strength Testing Tensile strength of a ferrous metal is tested by applying a longitudinal load to a sample of material and plotting the load against the resulting elongations on a graph.
Fatigue Strength Testing It is used to establish the stress level at which structural failure will occur. A specially shaped test piece is gripped at one end, while at the other end a ball race is fitted. A load is suspended from the ball race and the test piece is then rotated from the end at which it is held by an electric motor. Under the action of the overhung load, it is stressed in tension and compression once every revolution.
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The numbers of cycles are measured to determine fatigue failure. This number of cycles is then classified as fatigue life.
Airframe Fatigue Testing Aircraft are subjected to fatigue tests. The purpose of an airframe fatigue test is to provide key data that help design engineers identify the likelihood and causes of premature fatigue damage or wear on the airplane's structure and structural components. Aircraft are subjected to fatigue strength tests to determine fatigue life of components to help with maintenance programs. The picture is showing the Boeing 787 airframe fatigue test.
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Impact Resistance Testing Two tests called the Charpy and Izod impact tests are used to measure the impact resistance (or impact strength) of a metal. Both tests are mechanical tests in which a pendulum hammer (swinging through a fixed distance) fractures a standard size notched piece of material with one blow. The main difference between the Izod impact test and the Charpy impact test is that each one uses a different beam configuration (cantilevered configuration vs a three point beam configuration).
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Charpy V-notch (CVN) technique By far the most common impact testing method used today for metals is the Charpy V-notch impact test. With the Charpy V-notch (CVN) technique, the specimen is in the shape of a bar of square cross section with a V notch. The load is applied as an impact below from a weighted pendulum hammer that is released from a position h. The pendulum with a knife edge strikes and fractures the specimen at the notch. The pendulum continues its swing, rising to a maximum height h', which is lower than h. The energy necessary to fracture the test piece is directly calculated from the difference in initial and final heights of the swinging pendulum. The impact energy (toughness) from the Charpy test is related to the area under the total stress-strain curve. Pictured is a machined charpy specimen, a standard impact tester, and a computer controlled chiller unit capable of maintaining temperatures down to –50 degrees centigrade for reduced temperature impact testing.
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TOPIC 6.2.1 AND 6.2.2: AIRCRAFT MATERIALS - NONFERROUS PROPERTIES OF NONFERROUS METALS Much of the metal used on aircraft contains no iron. The term that describes metals which have elements other than iron as their base is nonferrous. Aluminium, copper, titanium, and magnesium are some of the more common nonferrous metals used in aircraft construction and repair.
Pure aluminium lacks sufficient strength to be used for aircraft construction. However, its strength increases considerably when it is alloyed, or mixed with other compatible metals. For example, when aluminium is mixed with copper or zinc, the resultant alloy is as strong as steel with only one third the weight. Furthermore, the corrosion resistance possessed by the Aluminium carries over to the newly formed alloy. Aluminium alloys are classified by their major alloying ingredient. The elements most commonly used for Aluminium alloying are: Copper Magnesium Manganese Zinc Based on the American National Standards Institute (ANSI) Standards H35.1 and H35.2, there are TWO main classes of aluminium alloys: Wrought alloys, rolled from an ingot or extruded from customer-specified shapes Cast alloys, poured as a liquid into a mould and cooled into a solid shape
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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Most aircraft parts are wrought aluminium alloys.
Wrought Aluminium Alloys Designation System Wrought aluminium alloys are identified by a four-digit index system.
Alloy Series
Principal Alloying Elements
1xxx 2xxx 3xxx 4xxx 5xxx 6xxx 7xxx 8xxx 9xxx
Min 99.00% Aluminum Copper Manganese Silicon Magnesium Magnesium and Silicon Zinc Other elements Unused series
1XXX series 1XXX series is the only exception to the wrought alloy designation system. The 1st digit indicates the minimum aluminium content is 99%, and there is no major alloying element. The second digit indicates modifications in impurity limits. If the second digit is zero, there is no special control on individual impurities. Digits 1 through 9, which are assigned consecutively as needed, indicate special control of one or more individual impurities. The 3rd and fourth digits provide the minimum aluminium percentage above 99%. Thus, 1030 would indicate 99.30% minimum aluminium without special control on individual impurities. The designations 1130, 1230, 1330, etc. indicate the same purity with special control on one or more impurities. Likewise, 1100 indicates minimum aluminium content of 99.00% with individual impurity control.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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2xxx through 9xxx Series The major alloying elements are indicated by the first digit, as follows: 2xxx Copper 3xxx Manganese 4xxx Silicon 5xxx Magnesium 6xxx Magnesium and silicon 7xxx Zinc 8xxx Other element 9xxx Unused series The second digit indicates alloy modification. If the second digit is zero, it indicates the original alloy. Digits 1 through 9, which are assigned consecutively, indicate alloy modifications. The last two digits have no special significance, serving only to identify the different alloys in the group (number has no significance but is unique).
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Aluminium And Alloys Series 1xxx - Pure Aluminium: Strain-hardenable High formability, corrosion resistance and electrical conductivity Electrical, chemical applications Representative designations: 1100, 1350 The 1xxx series represents the commercially pure aluminium, ranging from the baseline 1100 (99.00% min. Al) to relatively purer 1050/1350 (99.50% min. Al) and 1175 (99.75 % min. Al). Some, like 1350 which is used especially for electrical applications, have relatively tight controls on those impurities that might lower electrical conductivity. The 1xxx series are strain-hardenable, but would not be used where strength is a prime consideration. Rather the emphasis would be on those applications where extremely high corrosion resistance, formability and/or electrical conductivity are required.
Alloys within the 2xxx series utilize copper as the principle alloying agent. When aluminium is mixed with copper, certain metallic changes take place in the resultant alloy’s grain structure. For the most part, these changes are beneficial and produce greater strength. However, a major drawback to aluminium-copper alloys is their susceptibility to intergranular corrosion when improperly heat-treated. Most aluminium alloy used in aircraft structures is an aluminium-copper alloy. Two of the most commonly used in the construction of skins and rivets are 2017 and 2024.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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The 3xxx series alloys have manganese as the principle alloying element, and are generally considered non heat-treatable. The most common variation is 3003, which offers moderate strength and has good working characteristics.
The 4xxx series aluminium is alloyed with silicon, which lowers a metal’s melting temperature. This results in an alloy that works well for welding and brazing.
Magnesium is used to produce the 5xxx series alloys. These alloys possess good welding and corrosion resistance characteristics. However, if the metal is exposed to high temperatures or excessive cold working, its susceptibility to corrosion increases.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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If silicon and magnesium are added to aluminium, the resultant alloy carries a 6xxx series designation. In these alloys, the silicon and magnesium form magnesium silicide which makes the alloy heat-treatable. Furthermore, the 6xxx series has medium strength with good forming and corrosion resistance properties.
When parts require more strength and little forming, harder aluminium alloys are employed. The 7xxx series aluminium alloys are made harder and stronger by the addition of zinc. Some widely used forms of aluminium-zinc alloys are 7075 and 7178. The aluminium-zinc alloy 7075 has a tensile strength of 77 KSI and a bearing strength of 139 KSI. However, the alloy is very hard and is difficult to bend. An even stronger zinc alloy is 7178 which has a tensile strength of 84 KSI and a bearing strength of 151 KSI. 1KSI = 1 kilopounds / square inch (1 KIP) = 1,000 pound-force/square inch (PSI) For example, on B737-200 aircraft: Frames, stringers, keel and floor beams, wing ribs - Aluminium alloy 7075 (Aluminium and zinc) - High mechanical properties and improved stress corrosion cracking resistance. Bulkheads, window frames, landing gear beam - Aluminium alloy 7079 (Aluminium and zinc) Tempered to minimise residual heat treatment stresses. Wing upper skin, spars and beams - Aluminium alloy 7178 (Aluminium, zinc, magnesium and copper) - High compressive strength to weight ratio. Landing gear beam - Aluminium alloy 7175 (Aluminium, zinc, magnesium and copper) A very tough, very high tensile strength alloy. Wing lower skin - Aluminium alloy 7055 (Aluminium, zinc, magnesium and copper) Superior stress corrosion.
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8xxx series used for those alloys with lesser used alloying elements such as Fe, Ni and Li. Each is used for the particular characteristics it provides the alloys: Fe and Ni provide strength with little loss in electrical conductivity and so are used in a series of alloys represented by 8017 for conductors. Li in alloy 8090 provides exceptionally high strength and modulus (elasticity), and so this alloy is used for aerospace applications where increases in stiffness combined with high strength reduces component weight.
The 9xxx series alloys are unassigned at the time.
Clad Aluminium Alloy The clad surface greatly increases the corrosion resistance of an aluminium alloy. However, if it is penetrated, corrosive agents can attack the alloy within. For this reason, sheet metal should be protected from scratches and abrasions. In addition to providing a starting point for corrosion, abrasions create potential stress points.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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HEAT TREATMENT OF ALUMINIUM ALLOYS Heat treatment is a series of operations involving the heating and cooling of metals in their solid state. Its purpose is to make the metal more useful, serviceable, and safe for a definite purpose. By heat-treating, a metal can be made harder, stronger, and more resistant to impact. Heat-treating can also make a metal softer and more ductile. However, one heat-treating operation cannot produce all these characteristics. In fact, some properties are often improved at the expense of others. In being hardened, for example, a metal may become brittle. All heat-treating processes are similar in that they involve the heating and cooling of metals. They differ, however, in the temperatures to which the metal is heated and the rate at which it is cooled.
There are two types of heat treatments used on aluminium alloys. One is called solution heat treatment, and the other is known as precipitation heat treatment. Some alloys, such as 2017 and 2024, develop their full properties as a result of solution heat treatment followed by about 4 days of cooling, or aging, at room temperature. However, other alloys, such as 2014 and 7075, require both heat treatments.
Solution Heat-Treatment The process of heating certain aluminium alloys to allow the alloying element to mix with the base metal is called solution heat-treating. In this procedure, metal is heated in either a molten sodium or potassium nitrate bath or in a hot-air furnace to a temperature just below its melting point. The temperature is then held to within about plus or minus 10 degrees Fahrenheit of this temperature and the base metal is soaked until the alloying element is uniform throughout. Once the metal has sufficiently soaked, it is removed from the furnace and cooled or quenched. It is extremely important that no more than about ten seconds elapse between removal of an alloy from the furnace and the quench. The reason for this is that when metal leaves the furnace and starts to cool, its alloying metals begin to precipitate out of the base metal. If this process is not stopped, large grains of alloy become suspended in the aluminium and weaken the alloy. Excessive precipitation also increases the likelihood of intergranular corrosion. To help minimize the amount of alloying element that precipitates out of a base metal, a quenching medium is selected to ensure the proper cooling rate. For example, a water spray or bath provides the appropriate cooling rate for aluminium alloys. However, large forgings are typically quenched in hot water to minimize thermal shock that could cause cracking.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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Thin sheet metal normally warps and distorts when it is quenched, so it must be straightened immediately after it is removed from the quench. After the quench, all metals must be rinsed thoroughly since the salt residue from the sodium or potassium nitrate bath can lead to corrosion if left on the alloy.
Precipitation Heat Treatment Heat-treatable aluminium alloys are comparatively soft when first removed from a quench. With time, however, the metal becomes hard and gains strength. When an alloy is allowed to cool at room temperature, it is referred to as natural aging and can take several hours or several weeks. For example, aluminium alloyed with copper gains about 90 percent of its strength in the first halfhour after it is removed from the quench, and becomes fully hard in about four or five days.
An alloy’s aging process time can be lengthened or shortened. For example, the aging process can be slowed by storing a metal at a sub-freezing temperature immediately after it is removed from the quench. On the other hand, the aging process can be accelerated by reheating a metal and allowing it to soak for a specified period of time. This type of aging is identified by several terms such as artificial age-hardening, precipitation-hardening or precipitation heat treatment. This process develops hardness, strength, and corrosion resistance by locking a metal’s grain structure together.
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Naturally aged alloys, such as the copper-zinc-magnesium alloys, derive their full strength at room temperature in a relatively short period and require no further heat treatment. However, other alloys, particularly those with high zinc content, need thermal treatment to develop full strength. These alloys are called artificially aged alloys.
Annealing Annealing is a process that softens a metal and decreases internal stress. In general, annealing is the opposite of hardening. To anneal an aluminium alloy, the metal’s temperature is raised to an annealing temperature and held there until the metal becomes thoroughly heat soaked. It is then cooled to 500°F at a rate of about 50° per hour. Below 500°F, the rate of cooling is not important. When annealing clad aluminium metals, they should be heated as quickly and as carefully as possible. The reason for this is that if clad aluminium is exposed to excessive heat, some of the core material tends to mix with the cladding. This reduces the metal’s corrosion resistance.
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Aluminium Alloy Temper Designations Heat-treatable alloys have their hardness condition designated by the letter -T followed by one or more numbers. A list of these designations includes: T Solution heat-treated T2 Annealed (cast products only) T3 Solution heat-treated, followed by strain hardening. Different amounts of strain hardening of the heat-treated alloy are indicated by a second digit. For example, -T36 indicates that the material has been solution heat-treated and has had its thickness reduced 5 percent by cold rolling. T4 Solution heat-treated, followed by natural aging at room temperature to a stable condition. T5 Artificially aged after being rapidly cooled during a fabrication process such as extrusion or casting. T6 Solution heat-treated, followed by artificial aging (precipitation heat-treated). T7 Solution heat-treated and then stabilized to control its growth and distortion. T8 Solution heat-treated, strain hardened, and then artificially aged. T9 Solution heat-treated, artificially aged, and then strain-hardened. T10 Artificially aged and then cold worked.
Reheat Treatment A material which has been previously heat-treated can generally be heat-treated several times. As an example, rivets made of 2017 or 2024 are extremely hard and typically receive several heat treatments to make them soft enough to drive. The number of solution heat treatments allowed for clad materials is limited due to the increased diffusion of core material into the cladding. This diffusion results in decreased corrosion resistance. As a result, clad material is generally limited to no more than three heat treatments.
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NON HEAT-TREATABLE ALLOYS The non heat-treatable aluminium alloys are usually designated, therefore in the 1000, 3000, 4000, 5000 series. Their properties can be adjusted by cold work, usually by cold rolling. Commercially pure aluminium does not benefit from heat treatment since there are no alloying materials in its structure.
By the same token, 3003 is an almost identical metal and, except for a small amount of manganese, does not benefit from being heat-treated. Both of these metals are lightweight and somewhat corrosion resistant. However, neither has a great deal of strength and, therefore, their use in aircraft is limited to non-structural components such as fairings and streamlined enclosures that carry little or no load. Alloy 5052 is perhaps the most important of the non heat-treatable aluminium alloys. It contains about 2.5 percent magnesium and a small amount of chromium. It is used for welded parts such as gasoline or oil tanks, and for rigid fluid lines. Its strength is increased by cold working.
Strain Hardening Both heat-treatable and non heat-treatable aluminium alloys can be strengthened and hardened through strain hardening also referred to as cold working or work hardening. This process requires mechanically working a metal at a temperature below its critical range. Strain hardening alters the grain structure and hardens the metal. The mechanical working can consist of rolling, drawing, or pressing.
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Heat-treatable alloys have their strength increased by rolling after they have been solution heattreated. On the other hand, non heat-treatable alloys are hardened in the manufacturing process when they are rolled to their desired dimensions. However, at times these alloys are hardened too much and must be partially annealed.
Hardness Designations Where appropriate, a metal’s hardness, or temper, is indicated by a letter designation that is separated from the alloy designation by a dash. When the basic temper designation must be more specifically defined, one or more numbers follow the letter designation. These designations are as follows: F As fabricated. O Annealed, recrystallised (wrought materials only). H Strain hardened. H1 Strain hardened only. H2 Strain hardened and partially annealed. H3 Strain hardened and stabilized. The digit following the designations H1, H2, and H3 indicate the degree of strain hardening. For example, the number 8 represents the maximum tensile strength while O indicates an annealed state. The most common designations include: Hx2 Quarter-hard Hx4 Half-hard Hx6 Three-quarter hard Hx8 Full-hard Hx9 Extra-hard
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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MAGNESIUM AND ITS ALLOYS Magnesium is collected by electrolysis of sea water or brine but does not have the required strength in its pure form. Magnesium alloys are used for castings and in its wrought form is available in sheets, bars, tubing, and extrusions. Magnesium is one of the lightest metals having sufficient strength and suitable working characteristics for use in aircraft structures. It has a lower density compared with aluminium. It weighs only about 2/3 as much as aluminium
The drawbacks to using magnesium are: High susceptibility to corrosion - This is over come by treating the surface with chemicals that form an oxide film and exclude oxygen. Tendency to crack when formed – This is over come by heating the parts when forming such as hot dimpling thin material. In addition to cracking and corroding easily, magnesium burns readily in a dust or small particle form. For this reason, caution must be exercised when grinding and machining magnesium. If a fire should occur, extinguish it by smothering it with dry sand or some other dry material that excludes air from the metal and cools its surface. If water is used, it will only intensify the fire.
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Solution heat treatment of magnesium alloys increases tensile strength, ductility, and resistance to shock. After a piece of magnesium alloy has been solution heat-treated, it can be precipitation heat-treated by heating it to a temperature lower than that used for solution heat treatment, and holding it at this temperature for a period of several hours. This increases the metal’s hardness and yield strength.
The American Society for Testing Materials (ASTM) has developed a classification system for magnesium alloys that consists of a series of letters and numbers to indicate alloying agents and temper condition. The letter after the alloying elements and percentages simply identifies the sequence in which the specific alloy specification for the general alloy composition was registered with ASTM. Example: AZ31A is the first specification registered for the 3% Al - 1% Zn alloy of magnesium, AZ31B is the second alloy specification. The difference between the two is the limits on the minor elements/contaminants, which can impact corrosion performance or other performance factors
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TITANIUM AND ITS ALLOYS Titanium and its alloys are light weight metals with very high strength. Pure titanium weighs 0.163 pounds per cubic inch, which is about 50 percent lighter than stainless steel, yet it is approximately equal in strength to iron. Furthermore, pure titanium is soft and ductile with a density between that of aluminum and iron. Titanium is a metallic element which, when first discovered, was classified as a rare metal. However, in 1947 its status was changed due to its importance as a structural metal. In the area of structural metallurgy, it is said that no other structural metal has been studied so extensively or has advanced aircraft structures so rapidly. In addition to its light weight and high strength, titanium and its alloys have excellent corrosion resistance characteristics, particularly to the corrosive effects of salt water. However, since the metal is sensitive to both nitrogen and oxygen, it must be converted to titanium dioxide with chlorine gas and a reducing agent before it can be used. Titanium is classified as alpha, alpha beta, and beta alloys. These classifications are based on specific chemical bonding within the alloy itself. The specifics of the chemical composition are not critical to working with the alloy, but certain details should be known about each classification. Alpha alloys have medium strengths of 120 KSI to 150 KSI and good elevated-temperature strength. Because of this, alpha alloys can be welded and used in forgings. The standard identification number for alpha titanium is 8A1-lMo-1 V-Ti, which is also referred to as Ti-B-i-I. This series of numbers indicates that the alloying elements and their percentages are 8 percent aluminum, 1 percent molybdenum, and 1 percent vanadium. Alpha-beta alloys are the most versatile of the titanium alloys. They have medium strength in the annealed condition and much higher strength when heat treated. While this form of titanium is generally not weldable, it has good forming characteristics. Beta alloys have medium strength, excellent forming characteristics, and contain large quantities of high-density alloying elements. Because of this, beta titanium can be heat-treated to a very high strength. The grain size of titanium is refined when aluminium is added to the alloy mixture, however, when copper is added to titanium, a precipitation-hardening alloy is produced. Titanium added to high temperature nickel-cobalt-chromium alloy produces a precipitation-hardening reaction which provides strength at temperatures up to 1,500°F. Because of its high strength-to-weight ratio, titanium is now used extensively in the civilian aerospace industry. Although once rare on commercial aircraft, modern jet transports now utilize alloys containing 10 to 15 percent titanium in structural areas.
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NICKEL AND ITS ALLOYS Two commonly used nickel alloys are: Monel Inconel Monel contains about 68% nickel and 29% copper with small amounts of iron and manganese. Monel can be welded and has very good machining characteristics. Certain types of Monel, especially those containing small percentages of aluminium, are heat-treatable to tensile strengths equivalent to steel. Monel works well in gears and parts that require high strength and toughness, as well as for parts in exhaust systems that require high strength and corrosion resistance at elevated temperatures. Monel rivets are also available for aircraft structural applications.
Inconel contains about 80% nickel and 14% chromium with small amounts of iron and other elements. The alloys find frequent use in turbine engines because of their ability to maintain their strength and corrosion resistance under extremely high temperatures. For example Inconel can be found in nozzle supports, fan casings, and blades.
Inconel and stainless steel are similar in appearance and are frequently used in the same areas. Differentiating between the two must be done through a chemical test.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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COPPER AND ITS ALLOYS Neither copper nor its alloys find much use as structural materials in aircraft construction. However, due to its excellent electrical and thermal conductivity, copper is the primary metal used for electrical wiring. Of the several alloys that use copper as a base, the following alloys are the primary alloys found on aircraft: Brass: Brass is a copper alloy containing zinc and small amounts of aluminium, iron, lead, manganese, magnesium, nickel, phosphorous, and tin. Bronze: Bronze is a copper alloy that contains tin. A true bronze consists of up to 25% tin, and along with brass, is used in bushings, bearings, fuel metering valves, and valve seats. Beryllium: Beryllium copper is probably one of the most used copper alloys. It consists of approximately 97% copper, 2% beryllium, and sufficient nickel to increase its strength. Beryllium is used for diaphragms, precision bearings and bushings, ball cages, and spring washers.
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NONFERROUS METAL TESTING Once the heat treatment process has been carried out, the material must then be tested to insure that the desired qualities have been achieved. Some of the properties that would be tested for are: Hardness Tensile strength Fatigue strength Impact resistance Note – testing is carried out with a test piece consisting of the same material and thickness of component, which was heat treated with the component.
Hardness Testing Since the strength of most metals varies with hardness, it is often required to measure the hardness of a metal. The two most widely used methods of hardness measurement are the Brinell and Rockwell methods.
Brinell hardness tester The Brinell hardness tester uses a hydraulic force to impress a spherical penetrator into the surface of a sample. The amount of force used is approximately 3,000 kilograms for steel, and 500 kilograms for nonferrous metals. This force is hydraulically applied by a hand pump and read on a pressure gauge. When the sample is removed from the tester, the diameter of the impression is measured with a special calibrated microscope.
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The diameter of the impression is then converted into a Brinell number by using a chart furnished with the tester.
Alloy number
Hardness temper
Brmell number 500 kg load 10 mm ball
1100
O H18 O H16 O T6 O T6 O T4 O H36 O T4 T6 T6
23 44 28 47 5 135 45 105 47 120 47 73 30 55 95 135
3003 2014 2017 2024 5052 6061
7075
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Rockwell hardness tester The Rockwell hardness tester gives the same information the Brinell tester gives, except that it measures the depth to which the penetrator sinks into the material rather than the diameter of the impression. To use a Rockwell hardness tester, a sample is thoroughly cleaned, the two opposite surfaces are ground flat and parallel, and all scratches are polished out. The sample is then placed on the anvil of the tester and raised up against the penetrator. A 10-kilogram load, called the minor load, is applied and the machine is zeroed. A major load is then applied and the dial on the tester indicates the depth the penetrator sinks into the metal. Instead of indicating the depth of penetration in thousandths of an inch, it indicates in Rockwell numbers on either a red or a black scale. Rockwell testers use three types of penetrators: a conical diamond, a 1/16 inch ball, and a 1/8 inch ball. There are also three major loads: 60 kilograms, 100 kilograms, and 150 kilograms. The two most commonly used Rockwell scales are the B-scale for soft metals, which uses a 1/16 ball penetrator and a 100 kg major load, and the C-scale for hard metals, which uses the conical diamond penetrator and a 150 kg major load.
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Procedure for Rockwell Hardness Test: The indenter moves down into position on the part surface A minor load (F0) is applied and a zero reference position is established Major load (F1) is applied for a specified time period (dwell time) beyond zero The major load is released leaving the minor load applied The resulting Rockwell number represents the difference in depth (E - e) from the zero reference position as a result of the application of the major load
Tensile Strength Testing Tensile strength of a ferrous metal is tested by applying a longitudinal load to a sample of material and plotting the load against the resulting elongations on a graph.
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Fatigue Strength Testing It is used to establish the stress level at which structural failure will occur. A specially shaped test piece is gripped at one end, while at the other end a ball race is fitted. A load is suspended from the ball race and the test piece is then rotated from the end at which it is held by an electric motor. Under the action of the overhung load, it is stressed in tension and compression once every revolution. The numbers of cycles are measured to determine fatigue failure. This number of cycles is then classified as fatigue life. The Wohler Cantilever bending fatigue machine is shown as an example.
Airframe Fatigue Testing Aircraft are subjected to fatigue tests. The purpose of an airframe fatigue test is to provide key data that help design engineers identify the likelihood and causes of premature fatigue damage or wear on the airplane's structure and structural components.
The picture is showing the Boeing 787 airframe fatigue test.
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Impact Resistance Testing Two tests called the Charpy and Izod impact tests are used to measure the impact resistance (or impact strength) of a metal. Both tests are mechanical tests in which a pendulum hammer (swinging through a fixed distance) fractures a standard size notched piece of material with one blow. The main difference between the Izod impact test and the Charpy impact test is that each one uses a different beam configuration (cantilevered configuration vs a three point beam configuration).
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Charpy V-notch (CVN) technique By far the most common impact testing method used today for metals is the Charpy V-notch impact test. With the Charpy V-notch (CVN) technique, the specimen is in the shape of a bar of square cross section with a V notch. The load is applied as an impact below from a weighted pendulum hammer that is released from a position h. The pendulum with a knife edge strikes and fractures the specimen at the notch. The pendulum continues its swing, rising to a maximum height h', which is lower than h. The energy necessary to fracture the test piece is directly calculated from the difference in initial and final heights of the swinging pendulum. The impact energy (toughness) from the Charpy test is related to the area under the total stress-strain curve. Pictured is a machined charpy specimen, a standard impact tester, and a computer controlled chiller unit capable of maintaining temperatures down to –50 degrees centigrade for reduced temperature impact testing.
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TOPIC 6.3.1 COMPOSITES AND NON-METALLIC NON-METALLIC STRUCTURAL MATERIALS Non-metallic structural materials played a very important role in the early days of aviation. However, after the introduction of aluminium, non-metallic materials saw considerably less use. Today, aluminium is still the most widely used material in the construction and repair of aircraft. However, since its introduction, several new materials have come into use, many of which are spinoffs from the space program. These materials once again have the aviation industry taking a close look at non-metallic materials for use on aircraft.
WHAT IS A COMPOSITE STRUCTURE? The term composite is used to describe two or more materials that are combined to form a structure that is much stronger than the individual components.
A more contemporary example of composite material is the traditional dope and fabric aeroplane. In this instance, nitrate or butyrate dope is combined in proper proportions with grade A cotton fabric, producing a strong, light weight skin covering. The strength and simplicity of the dope and fabric aeroplane has endured through the years and is still a favourite airframe material for handcrafted classic and high-performance aerobatic aeroplanes.
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Composite structures differ from metallic structures in several ways: excellent elastic properties, ability to be customized in strength and stiffness, damage tolerance characteristics, and sensitivity to environmental factors. Consequently, composites require a vastly different approach from metals with regard to their design, fabrication and assembly, quality control, and maintenance.
One main advantage to using a composite over a metal structure is its high strength-to-weight ratio. Weight reduction is a primary objective when designing structures using composite materials.
In addition, the use of composites allows the formation of complex, aerodynamically contoured shapes, reducing drag and significantly extending the range of the aircraft. Composite strength depends upon the type of fibres and bonding materials used, and how the part is engineered to distribute and withstand specific stresses.
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COMPOSITE IN AIRCRAFT STRUCTURES Below image is simply to underscore the amount of composite used in A320 aircraft structure.
Composite materials are found in the manufacture of primary flight controls and high integrity structural repairs in aircraft.
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COMPOSITE ELEMENTS In aircraft construction, most currently produced composites consist of a reinforcing material to provide the structural strength, joined with a matrix material to serve as the bonding substance. In addition, adding core material saves overall weight and gives shape to the structure.
The three main parts of a fibre-reinforced composite are the fibre, matrix, and interface or boundary between the individual elements of the composite.
Reinforcing Fibres Reinforcing fibres provide the primary structural strength to the composite structure when combined with a matrix. Reinforcing fibres can be used in conjunction with one another (hybrids), woven into specific patterns (fibre science), combined with other materials such as rigid foams (sandwich structures), or simply used in combination with various matrix materials. Each type of composite combination provides specific advantages. Following are the five most common types of reinforcing fibres used in aircraft composites: Fibreglass (Glass Cloth) Aramid Carbon / Graphite Boron Ceramic
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Fibreglass (Glass Cloth) Fibreglass is made from small strands of molten silica glass (about 1260oC) that are spun together and woven into cloth. There are many different weaves of fibreglass available, depending on the particular application. Its widespread availability and its low cost make fibreglass one of the most popular reinforcing fibres. Fibreglass weighs more than most other composite Fibres, but has less strength. In the past, Fibreglass was used for non-structural applications; the weave was heavy and polyester resins were used, making the part brittle. Recently, however, newly developed matrix formulas have increased the benefits of using Fibreglass.
The two most common types of fibreglass are S-glass and E-glass. E-glass, otherwise known as “electric glass” because of its high resistivity to current flow, is produced from borosilicate glass and is the most common type of fibreglass used for reinforcement.
S-glass is produced from magnesia-alumina- silicate, and is used where very high tensile strength fibreglass is needed. Aramid In early 1970s, Dupont introduced Aramid, an organic aromatic-polymide polymer, commercially known as Kevlar. Aramid exhibits high tensile strength, exceptional flexibility, high tensile stiffness, low compressive properties, and excellent toughness. The tensile strength of Kevlar® composite material is approximately four times greater than alloyed aluminium. Aramid fibres are non-conductive and produce no galvanic reaction with metals. Another important advantage is its outstanding strengthIssue C: August 2009
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to-weight ratio; it is very light compared to other composite materials. Aramid-reinforced composites also demonstrate excellent vibration-damping characteristics in addition to a high degree of shatter and fatigue resistance.
A disadvantage to Aramid is that it stretches, which can cause problems when it is cut. Drilling Aramid can also be a problem if the drill bit grabs a fibre and pulls until it stretches to its breaking point. When cuffing Aramid fabrics, the material will look fuzzy if inappropriate tools are used. Fuzzy material left around fastener holes or seams may act as a wick and absorb moisture or other liquid contaminants such as oil, fuel, or hydraulic fluid. Liquid contaminates may deteriorate the resin materials in the composite structure, producing delamination. It is important to cut Aramid cloth correctly because even a slight amount of moisture will prevent Aramid from bonding properly. Fuzz around the drilled hole may also prevent a fastener from seating properly, which may cause joint failure. Carbon / Graphite Carbon Fibre, also known as graphite Fibre, is a very strong, stiff reinforcement. For many years, American manufacturers used the term graphite, while European manufacturers used the term carbon. Carbon correctly describes the Fibre since it contains no graphite structure. Regardless of what you call it, you order it by number. If you order Carbon #584 you will get the same weight and weave as if you order Graphite #584. It is the same material. Some structural repair manuals may call for Carbon #584 in one area, and Graphite #584 in another. Recently, Carbon has become the favoured term by both American and European manufacturers. There are still many structural repair manuals however, that use the term Graphite, so understanding that this material can be referred to in either way is still important to the maintenance technician.
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Advantages to carbon/graphite materials are in their high compressive strength and degree of stiffness. However, carbon fibre is cathodic while aluminium and steel are anodic. Thus, carbon promotes galvanic corrosion when bonded to aluminium or steel, and special corrosion control techniques are needed to prevent this occurrence. Carbon/graphite materials are kept separate from aluminium components when sealants and corrosion barriers, such as fibreglass, are placed at the interfaces between composites and metals. To further resist galvanic corrosion, anodize, prime, and paint any aluminium surfaces prior to assembly with carbon/graphite material. Carbon fibre composites are used to fabricate primary structural components, such as ribs and wing skins. Even very large aircraft can be designed with a reduced number of reinforcing bulkheads, ribs, and stringers, thanks to the high strength and high rigidity of carbon fibre composites. Carbon fibre is stronger in compressive strength than Kevlar, but it is more brittle. Boron Boron fibres are made by depositing the element boron onto a thin filament of tungsten. The resulting fibre is approximately .004 inch in diameter, has excellent compressive strength and stiffness, and is extremely hard. However, boron is not commonly used in civil aviation because it can be hazardous to work with, and is extremely expensive. In designing components that need both the strength and stiffness associated with boron, many civil aviation manufacturers are utilizing hybrid composite materials of Aramid and carbon/graphite instead of boron.
Ceramic Ceramic fibres are used where a high-temperature application is needed. This form of composite will retain most of its strength and flexibility at temperatures up to 2,200° F Tiles on the Space Shuttle are made of a special ceramic composite that dissipates heat quickly. Some firewalls are also made of ceramic-fibre composites. The most common use of ceramic fibres in civilian aviation is in combination with a metal matrix for high-temperature applications.
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FIBRE SCIENCE The selective placement of fibres needed to obtain the greatest amount of strength in various applications is known as fibre science. The strength and stiffness of a composite depend on the orientation of the plies to the load direction. A sheet metal component will have the same strength no matter which direction it is loaded. For example, a helicopter rotor blade has high stress along its length because of centripetal forces. If the blade is made of metal, the strength is the same in all directions, giving strength in directions that are not needed. If fabricated of composites, however, the blade may have the majority of fibres running through its length to give more strength in the direction in which the most stress is concentrated. These vectors of strength might be referred to as zero degree plies (to react to axial loads like those to which a rotor blade is subjected), 45° plies (to react to shear vectors), or 90° plies (to react to side loads). For example, if a wing in flight bends up as well as twists, the part can be manufactured so one layer of fibres runs the length of the wing, reducing the bending tendency, and another layer with the fibres running at 45° and at 90°, to limit the twist, Each layer may have the major fibres running in a different direction. The strength of the fibres is parallel to the direction the threads run. This is how designers can customize fibre direction for the type of stress the part might encounter. Figure below shows in flight the structure tends to bend and twist. The fibre layers are laid in a way to limit the forces, thereby customizing a part to the type of stresses encountered.
Fabric Orientation When working with composite fibres, it is important to understand the construction and orientation of the fabric because all design, manufacturing and repair work begins with the orientation of the fabric. Unlike metallic structures, the strength of a composite structure relies on the proper placement and use of the reinforcing fibres. Some of the terms used to describe fibre orientation are warp, weft, selvage edge and bias.
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Warp The warp of threads in a section of fabric run the length of the fabric as it comes off the roll or bolt. Warp direction is designated as 0°. There are typically more threads woven into the warp direction than the fill direction, making it stronger in the warp direction. Because warp is critical in fabricating or repairing composites, insertion of another colour or type of thread at periodic intervals identifies the warp direction. Marked plastic backings on the underside of pre-impregnated fabrics also identify the orientation of the warp threads. Pre-impregnated fabrics are pre-impregnated with resins by the manufacturer and later cured in the field. Weft/Fill Weft or fill threads of the fabric are those that run perpendicular (90 degree) to the warp fibres. The weft threads interweave with the warp threads to create the reinforcing cloth. Selvage Edge The selvage edge of the fabric is the tightly woven edge parallel to the warp direction, which prevents edges from unravelling. The selvage edge is removed before the fabric is utilized. The weave of the selvage edge is different from the body of the fabric and does not have the same strength characteristics as the rest of the fabric. Bias The bias is the fibre orientation that runs at a 45 degree angle (diagonal) to the warp threads. The bias allows for manipulation of the fabric to form contoured shapes. Fabrics can often be stretched along the bias but seldom along the warp or fill.
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Fabric Styles Fabrics used in composite construction are manufactured in several different styles: Unidirectional, Bi-directional, Multidirectional, and Mat Component designers can use any or all of these fabric styles, depending on the strength and flexibility requirements of the component part. Unidirectional Unidirectional fibre orientation is one in which all of the major fibres run in one direction, giving the majority of its strength in a single direction. This type of fabric is not woven together, meaning that there are no fill fibres. Occasionally, small cross threads are used to hold the major fibre bundles in place. However, the cross threads are not considered woven fibres.
Bi-directional/Multi-directional Bi-directional or multi-directional fabric orientation calls for the fibres to run in two or more directions. Bi-directional fabrics are woven with the warp threads usually outnumbering the weft, so there is usually more strength in the warp direction than the fill. When using bi-directional fabrics, it is important to align the warp threads when performing a repair, due to the differences in the strength properties of the warp and weft directions.
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Mats Chopped fibres that are compressed together are often called mats. These mats are typically used in combination with other woven or unidirectional layers of fabric. A mat is usually not as strong as unidirectional or bidirectional fabric, and is not commonly used in repair work. This type of mat is used extensively in the marine industry for the manufacture of Glass fibre boat hulls.
Fabric Weaves Fabrics are woven together in a number of weaves and weights. Woven fabrics are more resistant to fibre breakout, delamination, and damage than unidirectional materials. Because of the wide variety of uses and strength requirements, composite fabrics are available in many weaves. Some of them are: Plain weave Satin weave Twill weave Plain weave In this most simple weave pattern, warp and fill yarns are interlaced over and under each other in alternating fashion. The plain weave provides good stability, porosity and the least yarn slippage for a given yarn count.
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Satin weave In the satin weave, the warp floats or skips over as many as 12 fill yarns before being woven in. A satin weave is notable for its smooth surface created by the relatively long warp yarn floats. The warp yarn in a 5-harness satin float over 4 fill yarns is shown.
Twill weave Twill weaves repeat on three or more warp and fill yarns. Twill weaves have a distinctive diagonal line on the surface of the fabric. There are many variations on the twill and a 2x2 twill weave is shown in the diagram.
The most common weaves used in advanced composite aircraft construction are the plain and satin weaves.
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MATRIX SYSTEMS The matrix is a bonding material that completely surrounds the fibre, giving it extra strength. The strength of a composite lies in the ability of the matrix to transfer stress to the reinforcing fibres. An advanced composite uses various manufacturing techniques and newer matrix formulas with newer reinforcing fabrics.
Polyester resin is an example of an early matrix formula used with fibreglass for many nonstructural applications such as fairings, spinners, and trim. The old polyester/fibreglass formulas did not offer sufficient strength to be used to fabricate primary structural members; it can be somewhat brittle. The newer matrix materials display remarkably improved stress distributing characteristics, heat resistance chemical resistance, and durability. Most of the newer matrix formulas for aircraft are epoxy resins. Resin matrixes are two-part systems consisting of a resin and a catalyst or hardener, which acts as a curing agent. The term resin often times means both parts together, not just the resin. Many times a maintenance manual may use the term catalysed resin, meaning that the resin and the curing agent or hardeners have been mixed, but not necessarily cured.
Resin Matrix Systems Resin matrix systems are a type of plastic. Some companies refer to composites as fibre reinforced plastics. There are two general categories of plastics: Thermoplastic Thermosetting By themselves, these resins do not have sufficient strength for use in structural applications, however, when used as a matrix and reinforced with other materials, they form the high strength, lightweight structural composites used today. Thermoplastic Resins Thermoplastic resins use heat to form the part into the desired shape; one that is not necessarily permanent. If a thermoplastic is heated a second time, it will flow to form another shape. Two types of transparent thermoplastic materials are used for aircraft windshields and side windows. They are cellulose acetate and acrylic. Early aircraft used cellulose acetate plastic because of its transparency and light weight. However, it has a tendency to shrink and turn yellow and, therefore, has almost completely been replaced. Cellulose acetate can be identified by its slightly yellow tint and the fact that, if a scrap of it is burned, it produces a sputtering flame and dark Issue C: August 2009
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smoke. Another way to identify cellulose acetate is with an acetone test. When acetone is applied to cellulose acetate it softens.
Acrylic plastics are identified by such trade names as Lucite or Plexiglas, or in Britain by the name Perspex. Acrylic is stiffer than cellulose acetate, more transparent and, for all practical purposes, is colourless. It burns with a clear flame and produces a fairly pleasant odour. Furthermore, if acetone is applied to acrylic it leaves a white residue but remains hard.
Thermosetting Resins They are usually liquids or low melting point solids in their initial form. When used to produce finished goods, they are “cured” by the use of a catalyst, heat, or a combination of the two. Once cured, solid thermosetting resins cannot be converted back to their original liquid form. Unlike thermoplastic resins, cured thermosets will not melt and flow but will soften when heated (and lose hardness) and once formed they cannot be reshaped.
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At this time, 2 common structural airframe applications are: Polyester resins Epoxy resins
Polyester Resin Polyester resin, an early thermosetting matrix formula, is mainly used with fibreglass composites to create non-structural applications such as fairings, spinners, and aircraft trim. While fibreglass possesses many virtues, its greatest limitation lies in its lack of structural rigidity. Polyester resins give fibreglass cohesiveness and rigidity. However, polyester resin/fibreglass composites do not offer sufficient strength to fabricate primary structural members.
Like other plastics, polyester shrinks when cured. While this inherent characteristic helps in some ways, it hurts in others. In its pure form, polyester resin is thick and unmanageable. For this reason, a styrene monomer is added to polyester resin to thin it and make it more workable. If left alone, a mixture of polyester and styrene eventually hardens into a solid mass. To prevent this, inhibitors are added to resins to extend their working time. However, when the resin is to be used, a catalyst must be introduced to suppress the inhibitors and initiate the curing process. Furthermore, the resin’s curing time can be appreciably shortened by the addition of a measured amount of an accelerator. The amount of accelerator needed depends on the ambient temperature and the thickness of the resin layer.
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The actual cure of polyester resin occurs when a chemical reaction between the catalyst and accelerator generates heat within the resin. The less surface area there is for heat to escape, the more heat remains in the resin and the faster it cures. Therefore, when submitted to identical conditions, a thick layer of resin cures more rapidly than a thin layer.
Commonly catalysts used in polyester resins systems are: Methyl Ethyl Ketone Peroxide (M.E.K.P) Benzoyl Peroxide (BPO) Commonly accelerators used in polyester resins systems are: Cobalt Naphthenate (CoNap), also known as Cobalt Soap Dimethyl Amine (DMA) WARNING: Never mix accelerators directly with catalysts as they will violently react with each other, and catch on fire or explode, if not diluted by the resin first. Epoxy Resins Most of the newer aircraft composite matrix-formulas utilize epoxy resins, which are thermosetting plastic resins. Epoxy resin systems are well known for their outstanding adhesion, strength, and resistance to moisture and chemicals. They are also useful for bonding nonporous and dissimilar materials, such as metal parts to composite components. The manner in which the joints are designed, and how the surfaces are prepared, determines the quality of the bond. Each epoxy composite system is designed for a specific purpose. For example, a cowling may use an epoxy resin that will withstand high temperatures, while an aileron may require one made to withstand bending stresses. Both use epoxy resin systems but are very different in their chemical makeup, producing structures with different characteristics. Thus, not every type of epoxy resin is suitable for every type of structure or repair. Make sure to use the proper resin called for in the manufacturer’s repair manual. Never use an epoxy resin that is not approved for aircraft use, as the strength, flexibility, and moisture resistance qualities cannot be guaranteed. Some of the properties of epoxy which make it useful for bonded structures are its low shrinkage percentage, high strength-to-weight ratio, exceptional chemical resistance, and ability to adhere to Issue C: August 2009
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an almost endless variety of materials. Epoxy forms an extremely tight bond between glass and metal, however, if epoxy is used to bond glass to a metal window frame, the glass will crack from temperature changes because of the different expansion rates of the metal and the glass.
Epoxy resins are 2 part systems consisting of a base resin and a hardener - not catalyst, for curing. Hardener is mixed in larger amounts than a catalyst. Epoxies may be used in place of polyester resins for almost any application. They also have a long shelf life. Unmixed, epoxies generally keep for almost a year at 72° F. Once they are mixed, however, they have a very short pot life, which is the amount of time a catalysed resin remains in a workable state.
Adhesives Resins come in different forms. Resins used for laminating are generally thinner, to allow proper saturation of the reinforcing fibres. Others are used for bonding and are typically known as adhesives because they glue parts together. Adhesive resins and catalysts are available either in pre-mixed quantities or in separate containers. One of the most unique forms of adhesive is the film adhesive. This type of adhesive pre-blends the resin and catalyst on a thin film of plastic. Refrigeration of the film is required to slow the cure rate (the rate of change to its permanent form) of the resin. If left out at room temperature, the resin and catalyst will start to cure. In the freezer, the curing process slows down, lengthening the shelf life of the film. Adhesive films are often used to help bond patches to a repair area.
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Another form of adhesive is available in foam, which is primarily used to splice replacement honeycomb core segments to existing cores. When heat is applied to the adhesive, it foams up and expands into crevices. These types of foaming adhesives can also be used to permanently install fasteners.
Pre-Impregnated Materials Pre-impregnated fabrics, commonly known as “pre-pregs,” are fabrics that have the resin system already saturated into the fabric. Because many epoxy resins have high viscosity, it is often difficult to mix and work epoxy resins into the fabric to completely encapsulate the fibres. Fabrics are preimpregnated with the proper amount and weight of a resin matrix to eliminate the mixing and application details such as proper mix ratios and application procedures.
In addition to woven fabrics, manufacturers also pre-impregnate unidirectional materials. Preimpregnating unidirectional fabrics involves saturating the fibres with resin directly from individual spools of thread. Pre-preg materials offer a convenience over raw fabrics in many ways: The pre-preg contains the proper amount of matrix. It does not produce a resin rich or resin lean component if cured properly. The reinforcing fibres are completely encapsulated with the matrix. During hand lay-up, if a resin system has a high viscosity, or is very thick, it is sometimes difficult to get the resin into and around each individual fibre to produce the strongest cure. This is not a problem with the pre-preg fabrics. The technician does not have to worry about distorting the fabric weave while working the resin into the fabric.
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Pre-preg fabrics eliminate the need to manually weigh and mix components. In hand lay-up, the resin and curing agent must be properly weighed. If they are not weighed properly, too much resin or curing agent could result in a part that will not cure properly, causing an unairworthy condition. In many cases, pre-preg materials produce a stronger component or repair. This is because just the right amount of matrix-to fabric ratio has been applied and it has been mixed properly. However, the strength of a composite repair also varies greatly upon the manner in which the repair is accomplished and the manner in which it is cured. Pre-pregs were invented for the use of aircraft manufacturers to reduce the problems associated with completely wetting out the fabric with resin. It also saves time and reduces the problems associated with weighing and mixing the resins. Pre-preg fabrics also have disadvantages when working in a maintenance facility. Some of the disadvantages of working with pre-pregs are: Many pre-pregs must be stored in a freezer. This requirement must be met. If some pre-pregs are allowed to remain at room temperature for even a few hours, the resins/catalysts start their chemical reaction and begin to cure. The term “out of- freezer life” is the time that the material is actually out of the freezer and is being cut or transported. During this time, the resins are warming up to room temperature and will start to cure. While in the freezer, this chemical reaction is slowed down to allow a longer shelf life. Pre-preg fabrics usually have a limited shelf life even if kept in the freezer. Some pre-pregs must also be shipped in cold storage overnight, which adds to the expense. Many companies do not want to sell small quantities of a specific weave and resin system, so a full roll must be purchased. For those shops that do not work with large amounts of these materials, this is not cost effective. Pre-preg material is much more expensive than raw fabric that can be impregnated with the same type of resin system. This is especially true if the material exceeds its shelf life and must be discarded. Composite components and materials have not yet been standardized. When working with metal aircraft, any manufacturer can call for 2024-T3, and you would know what type of metal to use.
The resin systems in prepreg materials will rapidly degrade if not kept in cold storage. Moisture, dust or other contaminants will compromise bond durability. Warning: Bags containing pre-preg material should be opened only in a controlled environment and not open until the material has thawed to room temperature.
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Fillers Fillers, also known as thixotropic agents, are materials added to resins to control viscosity and weight, to increase pot life and cured strength, and to make the application of the resin easier. Fillers increase the volume of the resin, making it less dense and less susceptible to cracking, as well as lowering the weight of the material. Most fillers are inert and will not react chemically with the resin. Microballoons, chopped fibres, and flox are common types of fillers used in composite construction. Microballoons are small spheres manufactured from plastic or glass. Plastic microballoons must be mixed with a compatible resin system that will not dissolve the plastic. Glass microballoons, on the other hand, are not affected by resin mixtures, making them the primary thixotropic agent used in composite construction. The advantages to using microballoons are that they provide greater concentrations of resin in the edges and corners of the structure, they are less dense, which reduces the overall weight, and they provide lower stress concentrations throughout the structure. However, microballoons do not add strength to the composite structure. Chopped fibres and flox can also be added as fillers and have the advantage of adding strength to the cured mixture. Chopped fibres are made from any type of fibre cut into certain lengths, commonly 1/4 to 1/2-inch lengths. Flox is the fuzzy fibre taken from the fabric strands. Both chopped fibres and flox may be used when added strength is desired. For example, if a hole is accidentally drilled in the wrong place in a composite structure, filling the hole with a mixture of resin and flox provides more strength than pure resin. Using pure epoxy resin produces brittle and heavy plugs.
Metal Matrix Composites A metal matrix composite incorporates metal in the matrix, instead of a plastic resin or ceramic material. Metal matrices provide specific stiffness, improved fatigue, strength, and wear resistance, along with improved thermal characteristics. However, they are not readily used in the aviation field presently; they are still in the experimental stage. Several considerations being analysed are the potential reinforcement, matrix corrosion reactions, and thermal stresses due to the mismatch of thermal expansions between the reinforcements and the matrix.
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A large difference in melting temperature between the matrix and the reinforcements may result in matrix creep while the reinforcement remains elastic.
Types of Fibre-Reinforced Composites Several methods of fibre-reinforced composite construction are in use today. The two main methods used are laminated and sandwich construction. Laminated Composites The uses of structural laminations of fibreglass, paper, and linen have been around for some time. As knowledge of composites has evolved, materials made of Aramid (Kevlar®), carbon/graphite, boron, and ceramic fibres have been developed. Laminate composites consist of two or more layers of reinforcing material bonded together and embedded in a resin matrix. Laminated composites are built up to desired thicknesses by using multiple layers of reinforcing fabrics. Interply Hybrid Laminates Interply hybrid composites consist of two or more layers of different reinforcing material laminated together. Blending different advanced composite fabrics in a laminate can achieve the proper balance of stiffness, strength, and weight for a particular application. For example, Kevlar® may be combined with carbon to produce a structure that merges the flexibility, lightweight, and impact resistance of Kevlar® with the stiffness of carbon. The figure illustrates an interply-hybrid laminate that consists of layers of carbon, fibreglass, and Kevlar.
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This type of laminate has the toughness of Kevlar, the stiffness of carbon, and the heat resistance of fibre-glass. Sandwich Composites Sandwich construction consists of two or more laminated face sheets bonded to each side of a relatively thick, lightweight core. Sandwich composites offer high strength to weight ratios as compared to solid laminated structures. As previously discussed, there are several types of core materials available for use in sandwich construction. Each offers its own advantage and unique strength and rigidity characteristics. In general, the strength of the sandwich composite varies with the thickness and type of core material. The use of core materials in composite construction dramatically increases the strength of a structure. The core material is essentially sandwiched between two or more face sheets. The advantages of a sandwich structure can be shown by comparing a four-layer solid fibreglass laminate to that of a foam-core sandwich structure that is four times as thick. The sandwich composite incorporates two layers of fibreglass on the top and bottom of a foam core. In this arrangement, the part becomes 37 times stiffer than the solid fibreglass laminate, and ten times stronger, with only a 6% increase in weight.
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Core Materials Core materials are the central members of an assembly and are used extensively in advanced composite construction. When bonded between two thin face sheets, it provides a rigid and lightweight component. Composite structures manufactured in this manner are sometimes referred to as sandwich construction. A core material gives a great deal of compressive strength to a structure. For example, the sheet metal skin on a rotor blade has a tendency to flex in flight. This constant flexing causes metal fatigue. A composite blade with a central core material provides uniform stiffness throughout the blade and eliminates most of the flexing associated with metal blades. The two most common types of core materials utilized in sandwich construction are honeycomb and foam cores. In addition, wood cores are also occasionally used in composite construction.
Honeycomb Cores Honeycomb core materials consist of the six-sided shape of a natural honeycomb, which provides a core with a very high strength-to-weight ratio. Manufacturers construct honeycomb cores from aluminium, Kevlar®, carbon, fibreglass, paper, and steel. Nomex, a paper impregnated material, is also widely used as an advanced composite core material.
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The ribbon direction of a honeycomb core is the direction in which the honeycomb can be pulled apart. Pulling one side of the honeycomb that is perpendicular to the ribbon direction separates it, revealing the ribbon direction. If the pull is parallel to the ribbon, it is nearly impossible to tear the honeycomb. It is important to line up the ribbon direction of the replacement honeycomb core with that of the original when performing a repair honeycomb core repair to ensure consistent structural strength along with uniform compressive strength. Foam Cores There are many different types of foam core materials available, depending on the specific application. Foam core materials offer different densities and temperature characteristics for highheat applications and fire resistance. When using foams in a repair operation, it is important to use the proper type and density. Always refer to the manufacturer’s repair guidelines for recommended materials and procedures. Styrofoam, urethane foam, poly vinyl chloride (PVC), and strux are several common types of foam cores used in aircraft composite construction.
Styrofoam is commonly used on home-built aircraft and should only be used with an epoxy resin. Polyester resins dissolve Styrofoam. Do not confuse aircraft-quality Styrofoam with the type of Styrofoam used to make Styrofoam cups. Styrofoam cups use foam with large cell configurations that can not be used for structural applications. Aircraft-quality Styrofoam is comprised of smaller cells, which produce a much stronger core material.
A heated cutting wire can be used to cut an airfoil-shaped part from a block of Styrofoam. A hot wire cutter consists of a nichrome wire that is heated electrically. Technicians typically make the tool by stretching the wire in a frame. Attach a template to each end of the foam to provide a Issue C: August 2009
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uniform cut. Monitor the progress of the cut by making sure the increment marks, placed at each end of the foam, are crossed simultaneously, resulting in a smooth and even surface.
Urethane foam can be used with epoxy or polyester resins. However, urethane cannot be cut with a hot wire. Subjecting urethane foam to high heat produces a hazardous gas. Instead of a hot wire cutter, urethane is cut with a number of common tools. Knives are typically used to rough out the shape, and another piece of urethane foam is used to sand the piece to its desired size and shape. Other foam core materials include poly vinyl cub- ride (PVC), and styx (cellular, cellulose acetate) foam. PVC foam can be used with either polyester or epoxy resins and cut with a hot wire. Strux foam is commonly used to build up ribs or other structural supports. Storage conditions for core materials: Dust free environment Humidity controlled environment Stored flat and separately on racks Remotely from any contamination sources Moisture, dust or other contaminants will compromise bond durability If the storage area is not humidity controlled, core materials may have to be dried out before use.
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Composite Materials Time Terms Shelf Life This is the time span that a product will remain useful. This should be listed on the label. Temperature during storage will affect the shelf life. Pot life and Gel time Are strictly related to the activity of the catalyst. This is governed both by the proportion of the catalyst and the ambient temperature. For a given proportion of catalyst, the higher the air temperature the shorter will be the pot life and gel time of the resin. Hardening time Can vary a lot depending on the size and thickness of the moulding, and also the proportion of resin present. It will also again be affected by the air temperature. Maturing time Is the further period of time over which the moulding will continue to gain hardness and, eventually, complete stability. When fully matured, the moulding will have its maximum strength, hardness, chemical resistance and stability.
Working With Resins and Catalysts The matrix formula for most advanced composites is very exacting. A slightly improper mix ratio can make a tremendous amount of difference in the strength of the final composite. Because the mixing procedures are so important, they are always included with the resin containers. The aircraft structural-repair manual also outlines proper mixing procedures.
Manufacturers often produce pre-measured matrix packages. The advantage to using prepackaged resin systems is that they eliminate the weighing process and therefore remove the possibility of a mixing ratio error.
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The resin and catalyst are divided into separate containers that are attached on one end. When ready for use, the partition, which separates the resin from the catalyst, is broken to allow the two to mix. Still within the package, the resin/catalyst combination is mixed together by squeezing and kneading the package to thoroughly blend the mixture. When completely mixed, the package is cut with scissors and the resin dispensed. Disposable cartridges that store, mix, and apply two-component materials are also available and convenient to use. They are available in many sizes and can be tailored to specific uses. Like the pre-measured packages, cartridges also eliminate mixing ratio errors. To use epoxy cartridges, the seal that separates the two components must be broken with a plunger. The materials are then mixed together by moving the plunger in a twisting and up-anddown motion to thoroughly mix the resin and catalyst. The label describes how many strokes are required to give a thorough mix. A needle or syringe may then be installed onto the end of the cartridge, and the resin dispensed. Be sure to check the cartridge part number, shelf life expiration date, and any special instructions.
Dispose of resins Dispose of cured polyester resin /epoxy products I.A.W local regulations. Disposal of time expired polyester resin systems can be accomplished by mixing with appropriate catalyst and then disposing of cured product I.A.W local regulations. Disposal of “time expired” two part epoxy resin systems by mixing the two components together and disposing of cured product I.A.W local regulations. Do not dispose of unmixed, uncured polyester resin /epoxy into general waste bins. Do not dispose of catalyst soaked rags into general waste bins as rags could spontaneously combust. Contact local Hazardous material collection agencies for collection and disposal of large quantities of uncured product, accelerators and catalysts.
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COMPOSITE MATERIALS SAFETY CONSIDERATIONS Safety is always important when working with composite materials. Many accidents have occurred because of the improper usage and handling of composite materials. Before working with any composite resin or solvent, it is important to know exactly what type of material you are using and exactly how to use it.
Material Safety Data Sheets (MSDS) Material safety data sheets contain information on hazardous ingredients, health precautions, flammability characteristics, ventilation requirements, spill procedures, information for health professionals in case of an accident, along with transportation and labelling requirements. MSDS must be available for review in the shop where the specific material is stored and used. Review the MSDS and become familiar with the specific types of materials you come in contact with before you begin working with the materials.
Personal Protection Some materials cause allergic reaction and some people are more sensitive to certain materials than others. Therefore, it is imperative to protect your skin from contact with composite matrix materials. The most effective way to provide skin protection is by the use of protective gloves, respirators, face shields, safety glasses, and shop coats. Protective gloves are produced from many types of material. Make sure you review the MSDS to find out what materials do not react with the composite materials you are using. For example, natural rubber gloves disintegrate when exposed to certain types of epoxy resins. In any case, do not reuse; replace safety gloves after heavy use.
If any materials come in contact with your skin, remove the material immediately and be sure to wash the area thoroughly. In addition, wash your hands before and after working with the materials, and before eating or smoking. Many composite chemicals are irritants and may cause serious skin inflammation and irritation. There are special types of epoxy cleaners available that break down resins without drying or reacting with the skin. Again, be sure to check the MSDS before working with materials that may be potentially hazardous. Always work in well-ventilated areas when working with resins or solvents. Some resins are toxic enough to cause difficulty breathing and, in some cases, severe allergic reactions. Wear a respirator when working with, mixing, and applying resins, solvents or any other hazardous chemical, and keep contaminated hands away from your mouth. Do not ever ingest any composite chemical, because some are fatally poisonous. Issue C: August 2009
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Some of the solvents and matrix components can cause permanent blindness within a few seconds after contact with the eye. Goggles, which can be worn alone or in combination with prescription glasses, provide complete eye protection against front and side impact hazards, chemical liquid splashes, and dust. If you should splash any epoxy resin or solvents in your eye, rinse the eye out immediately, report the accident to your supervisor, and seek medical help.
Very serious eye accidents have occurred when people did not take the warnings seriously. If you get any substance in your eye, do not wait to seek medical attention. If the substance is left in the eye for a prolonged period of time, or overnight, the damage to the eye can become more severe. Don’t take chances. Tell your supervisor and seek medical help immediately. Some resins, hardeners, and solvents may make you go blind. Certain materials can cause allergic reactions when they contact the skin. Some people are more sensitive to these materials than others. Using rubber gloves is the most effective way to provide skin protection with these chemicals. These gloves should be replaced after heavy use. Remove any splashed resin from your skin immediately. Wash hands thoroughly before and after work, before eating or smoking, and before putting on gloves.
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Respiration and Ingestion You must have the proper ventilation when working with any resins or solvents. Additionally, some resins are sufficiently toxic as to require you to wear a respirator when working with them. To alleviate respiratory issues, some shops provide a ventilated mixing booth. However, once the chemicals have been mixed, it is often necessary to apply the resin in an unventilated area or otherwise expose yourself to the chemical fumes. In such instances, it is important that respirators are used once the mixed resins are removed from the mixing booth.
Protect exposed skin, use eye and respiratory protection and wear gloves when weighing, mixing and applying any resin system.
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Fire Protection Many solvents and resins are flammable, making it important to always work with and store these types of materials in well-ventilated areas. Keep all resins away from heat and open flames. Follow the safety recommendations of the manufacturer explicitly to reduce the chance of a fire. The following fire-safety requirements will reduce the fire danger in the shop: Eliminate all flames, smoking, sparks, and other sources of ignition from areas where solvents are used. Use non-spark-producing tools. Ensure that all electrical equipment meets the applicable electrical and fire codes. Keep flammable solvents in closed containers. Provide adequate ventilation to prevent the build-up of vapours. Statically ground the aircraft and any repair carts in use. Never unroll bagging films or other materials around solvents, to reduce the chance of static electricity. Never store or use solvents in any area when sanding.
Solvent Safety Tips There are many types of solvents used in composite construction today. Solvents are mainly used for cleaning purposes in composite construction. However, most solvents are flammable and must be used with the highest degree of safety in mind. Methyl-Ethyl-Ketone (MEK) and acetone are two common solvents used in composite construction. MEK is mainly used for cleaning dust, grease, and mold release agents from composite components. Always use protective gloves and goggles when using it. MEK is an excellent cleaner but also a carcinogen. It can be absorbed directly into the bloodstream through the skin and the eyes. Acetone is used for general equipment and tool cleanup, in addition to cleaning the composite parts after sanding as a pro-bond preparation. Follow the manufacturer’s recommendations when choosing the proper solvent.
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These safety guidelines should be followed when using all solvents and matrices: Do not use solvents in any area that may create a static charge. Do not pour solvents onto the part. Instead, use a solvent-soaked soft cloth to apply solvents to the working surface. Use solvents in a well-ventilated area and avoid prolonged breathing of the vapors. Wear gloves when applying solvents to protect the skin from drying out. Never use solvents to clean skin. Use suitable epoxy cleaners that are less dangerous. Wear goggles when pouring and working with solvents. Store solvents in the original containers so they can be readily identified.
Materials Storage For safety, read and follow the manufacturer’s instructions closely when handling and storing composite materials. Read the labels on containers for all information on handling, storage, and safety precautions. Improperly stored adhesives, resins, or prepregs may result in structurally unsafe aircraft components.
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COMPOSITE INSPECTION Today’s composite inspection techniques and non-destructive testing (NDT) methods typically involve the use of multiple methods to accurately determine the airworthiness of the structure. Fortunately, many metal inspection and NDT methods transfer to composite applications. Composite structures require ongoing inspection intervals along with non-scheduled damage inspection and testing. When a composite structure is damaged, it must first be thoroughly inspected to determine the extent of the damage, which often extends beyond the immediate apparent defect. Proper inspection and testing methods help determine the classification of damage, which is, whether the damage is repairable or whether the part must be replaced. In addition, classifying the damage helps to determine the proper method of repair. The manufacturer’s structural repair manual outlines inspection procedures, damage classification factors, and recommended repair methods.
Visual Inspection Visual inspection is the most frequently used inspection method in aviation. Ideally, pilots, ground crew, and maintenance technicians visually inspect the aircraft on a daily basis. This method of inspection is generally used to detect resin-rich areas, resin starvation, edge delamination, fibre break-out, cracks, blistering, and other types of surface irregularities. A strong light and magnifying glass are useful tools for visual inspection. In extremely critical cases, a small microscope is helpful in determining whether the fibres in a cracked surface are broken, or if the crack affects the resin only.
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Tap Testing It is an effective way of discovering if a finished job / repair has a good bond line or it has air pockets or other inclusions that may have caused a disbond.
Ultrasonic Inspection Ultrasonic inspection is the most common instrumental NDT method used on composites today. An ultrasonic tester is useful for detecting internal damage such as delaminations, core crush, and other subsurface defects. Two common methods of ultrasonic testing include the pulse echo and through transmission methods.
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In the pulse echo method, the tester generates ultrasonic pulses, sends them through the part, and receives the return echo. The echo patterns are displayed on an oscilloscope. An advantage to the pulse echo method is that it only requires access to one side of the structure. However, nearsurface defects do not readily allow sound to pass through them, making it difficult to detect defects located under the first defect. The pulse echo method works well on laminates because they do not reduce the magnitude of sound waves as much as a bonded core structure. The “through transmission” method uses two transducers. One transducer emits ultrasonic waves through the part and the other receives them. Defects located at multiple levels throughout the structure are more easily detected because the receiver, located on the backside of the part, receives the reduced amount of sound waves that pass through the defects. The ratio of the magnitudes of sound vibrations transmitted and received determines the structure’s reliability. Testing bonded-core structures usually requires the through transmission method due to the fact that sound waves reduce in magnitude as they travel through the sandwich structure. To effectively test this type of structure, the use of a receiver on the backside of the part dramatically increases the likelihood of detecting a defect.
Radiographic Inspection Radiography or x-ray inspection is used to detect differences in the thickness or physical density when compared to the surrounding material of a composite. It can be used to detect surface as well as internal cracks. Radiography also detects entrapped water inside honeycomb core cells. In addition to detecting the actual defect, it can also detect the extent and size of the damage, unlike ultrasonic or tap testing. X-ray inspection will also detect foreign objects in the composite structure if the object’s density is different from the composite structure.
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Thermography Thermography locates flaws by temperature variations at the surface of a damaged part. Heat is applied to the part and the temperature gradients are measured using an infrared camera. Thermography requires knowledge of the thermal conductivity of the test specimen and a reference standard for comparison purposes.
Dye Penetrants Dye penetrant successfully detects cracks and other defects in metallic surfaces, but should not be used on composite structure unless called for by the manufacturer.
If a dye penetrant is used on the composite structure and allowed to sit on the surface, the wicking action of the fibers may absorb the penetrant. Absorbed penetrant does not allow fibers to bond to new material. The entire area affected by the dye penetrant would have to be removed before new patches could be applied, which could extend the damage to a size that would make the part nonrepairable. Issue C: August 2009
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COMPOSITES DAMAGE Depending on the manufacturer of the Aircraft, classifications of damage is usually placed in one of three categories: Negligible - damage that may be corrected by a simple repair procedure with no restrictions on flight operations Repairable - damage to the skin, bond or core that cannot exist without placing restrictions on flight operations, but can be repaired Non-repairable - self explanatory, damage to the structure or component that can not be repaired (component must be replaced) Areas of condensation remaining on a structure after the aircraft has warmed up following cold soaking at altitude might be an indication of water ingress and should be investigated.
Cosmetic Defects A cosmetic defect is a defect on the outer surface skin that does not involve damage to the structural reinforcing fibres.
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Impact Damage Impact damage occurs when a foreign object strikes the part. The degree of damage may range from slight to severe. The most common cause of impact damage is careless handling during transportation or storage, or standing parts on their edge without adequate protection. The thin face sheets on a sandwich panel are very susceptible to impact damage. An area that has been subjected to impact damage should also be inspected for delamination around the impacted area. Nicking, chipping, cracking, or breaking away pieces of the edge or corner can also be caused from improper handling.
Delamination Delamination is the separation of fabric layers of material in a laminate. Delamination can occur with no visible indications from the outer skin. To compound the problem, delamination often accompanies other types of damage, particularly impact damage. This damage occurs as the result of several causes, including impact, moisture in the fabric, or lightning strikes. Disbonds The term disbond is defined as a separation of the composite material from another material to which it has been adhesively bonded. This is different to a delamination which refers to a similar separation between any plies or layers of the composite. Separation between the skin and core of a composite sandwich structure is separately referred to as a core disbond. A disbond may be the consequence of poor adhesion, service loading or impact damage. The disbond may not be visible externally and if tight or weakly bonded may be difficult to detect using NDE methods. The latter is known as a kissing bond. Disbonding is particularly important to avoid in joins such as end connections. Cracks Cracks can occur in composite structures just as in metallic ones. Cracking is a common form of damage in composites arising in manufacture or under service conditions. Cracking has a significant effect on the integrity of the composite, allowing environment ingress and damage to extend under service loading. Cracking is often associated with the final stages of in-service failure.
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Sometimes they can be detected visually, other times they may require more advanced methods of non-destructive inspection (NDI). A thorough inspection should be made to determine the extent of each crack.
Resin Matrix Damage It can be caused by many things, such as fire or excessive heat, UV rays, paint stripper, impacts etc.
Water and Aircraft Fluid Intrusion This is especially a problem with honeycomb cores. Causes weight gain, contamination of bond joints, corrosion in aluminium honeycomb, and disbonds if the water freezes and expands. Water intrusion is a very common problem with high temperature repairs. The heat of curing causes the trapped water to turn to steam, disbonding face sheets around the repair.
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It is a progressive way to inadvertently convert a small area of damage into a large one.
Hole Damage Holes may occur from impact damage, over-torquing fasteners, or as a result of fastener pullthrough. Holes drilled in the wrong location, wrong size, or wrong number of holes drilled can also be classified as hole damage. Holes caused by a lightning strike may burn off resins, leaving bare cloth. Tiny holes, known as pin holes, in the skin surface are not easily detected; however, they could lead to more extensive damage. If moisture is allowed to get into the core structure, along with the airflow over the part, it could cause a small delamination, which could grow into a very large delamination.
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COMPOSITES REPAIRS The exact procedures for repair of various laminated composite structures depend partly on the type of damage incurred. The damage can range from a relatively simple surface scratch, to damage completely through all internal plies and core honeycomb material. There are four basic types of composite repairs: Bolted metal or cured composite patches Bonded metal or cured composite patches Resin injection Laminating new repair plies to the damage Bolted and bonded surface patches are not usually recommended due to the fact that these types of patches do not restore the strength characteristics of the original structure. A bolted or bonded patch that is attached to the surface also causes undesirable aerodynamic changes. Resin injection repairs are used to fill holes or voids. They are accomplished by injecting resin into the hole of a damaged area using a needle and syringe. This type of repair is usually done on nonstructural parts. The injected resin does not restore the original strength, and, in some cases, expands the delamination. The most desirable type of permanent repair to composite structure is to laminate new repair plies in the damaged area. This type of repair involves removing the damaged plies, and laminating on new ones.
Composites Damage Assessment and Repair Process Below are the basic steps: Find the damage - clean damaged area and remove surface coatings Assess the extent of the damage - use visual and NDI techniques and then mark out the repair Define the repair procedure Fabricate the repair Prepare the repair site Lay up/install the repair Cleanup the repair site Inspect repair site for structural integrity – may use NDI again Document repair Issue C: August 2009
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Mechanically Fastened Repairs with Pre Cured Patches At times, the facilities and bagging equipment are not available to produce a proper composite repair. In this case, a temporary repair made of a pre-cured patch inserted with blind fasteners may be used. However, this type of repair does not produce a structure with the same strength as the original, and it may cause vibration because it is not a flush repair. If composite patches are required, kits with pre-cured patches may be available. Pre-cured patches come in several sizes and are produced to have the fibers of each layer in the correct orientation.
In addition, some manufacturers offer various sizes of core materials that are bonded to pre-cured laminates. The technician can route out the damaged area and simply insert this type of core and Issue C: August 2009
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laminate patch. This type of repair may have a type of adhesive pre-applied to help it bond. These types of patches are usually stabilized using some type of mechanical blind fastener, which is drilled through the patch and into the original part. The problem with using any type of rivet is that they have a tendency to crush the core and produce delamination. These types of repairs must be performed using the correct type of fasteners. Hole expanding fasteners such as MS20470 rivets should not be used in composite structures because of the possibility of causing damage. Impact damage and delamination may occur due to the pressure of the rivet gun and bucking bar and the expansion of the rivet. In addition, you must also make sure metallic fasteners will not react with the composite and cause galvanic corrosion. For example, metal fasteners used with carbon/graphite composites must be made of corrosion resistant steel or titanium to prevent this electrolytic action.
Potted Repairs Potted repairs use a filler to complete the composite repair process. They provide an alternative to installing a core material plug but do not provide as much strength as a core material. Filling a hole with a resin and filler mixture adds weight and decreases the flexibility of the part. However, many structural repair manuals still list the potted repair as a viable repair for composite structures, Most potted repairs are appropriate for honeycomb core sandwich structures with damage up to one-inch in diameter. In some cases, it is permissible to drill a small hole into a delaminated area and inject resin to strengthen the part. A typical potted repair procedure requires the technician to: Clean the damaged area. Sand out the delaminated area. Fill the core area with a resin and mixture. Prepare and install repair patches. Apply pressure and cure. Refinish the part.
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Injection Repairs In some cases, internal delamination is minor enough to repair using a potting compound. It can sometimes be repaired by simply injecting resin into the cavity that was caused by the ply separation. If the delamination is severe enough, it must be removed and repaired or replaced; always check the manufacturer’s repair limitations.
A typical delamination injection repair procedure for minor delamination follows. Clean the surface with an approved solvent. Outline the void area and mark the injection hole locations. Drill two .060-inch holes into the disbonded area taking care not to drill through the part. Inject mixed resin into one hole allowing air to vent from the other Clean excess resin from the surface of the part. Cure according to the manufacturer’s instructions. Minor edge delamination can sometimes be repaired by injecting resin into the delamination, clamping the edge and allowing the resin to cure.
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In addition, edges that have been damaged by crushing or puncture can be repaired by scarf cutting, and installing new plies.
Laminate Damage to One Surface This type of repair calls for the removal and replacement of the damaged laminate plies. Fiber damage to one side of the surface that does not completely penetrate the part may be repaired as follows: Prepare the surface by removing the paint and cleaning. Remove the damage by scarf or step-cutting the plies. Mix the resin and prepare the bonding patches. Apply pressure and cure the part. Re-finish the part.
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Laminate Damage through the Part Damage that runs through all of the laminate layers can be repaired in several ways depending on the number of plies, the location of the damage, and the size of the damage. Check the manufacturer’s repair manual for specific repair limitations regarding each type of damage. Repairable damage can be fixed in several different ways. The damage can be repaired using a step-cut that starts from one side of the part to the other, or, in the case of thicker laminate structures, repaired by step-cutting from both sides and meeting in the middle. View A illustrates a step-cut repair that runs from one side of the part to the other. In addition, a surface patch and backing plate with a one-inch overlap are applied to both surfaces of the repair. View B illustrates a modified step-cut repair. The step-cuts are started from both sides and meet in the middle. This type of repair reduces the size of the patch when performed on thicker laminates. Using view B as an example, the modified two-sided step-cut repair results in a patch that is four inches in diameter. If the one sided step-cut repair was performed on this same five- layer laminate, the patch would be five inches in diameter.
SANDWICH STRUCTURE REPAIRS Sandwich structures are vulnerable to impact and puncture damage primarily because these types of structures usually incorporate relatively thin face sheets. Because the face sheets of sandwich structures are relatively thin, delaminations commonly occur at the point where the face sheet bonds to the core material. Puncture damage may be repaired in several different ways depending on the size, extent, and location of the damage. Two of the more common types of sandwich structure repair are described below. Puncture Repair Honeycomb Core Repairs
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Puncture Repair Small punctures that penetrate one side and into the core material may be repaired using a resin and filler mixture. Check the repair manual damage limitations before proceeding with this type of repair. Generally, small punctures can be repaired using the following procedure: Determine the extent of the damage and check the repair limitations. Vacuum out the hole. Prepare the resin and filler (milled glass fibers). Work the resin/filler mixture into the hole. Cure the resin in accordance with the manufacturer’s instructions. Sand the surface with fine sandpaper. Prepare the surface for finishing using an approved solvent. Re-finish the part.
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Honeycomb Core Repairs As discussed previously, potted repairs may be made to areas of damage of up to one-inch diameter. If the damaged area is larger than an inch or in a more critical area, a balsa wood or composite honeycomb plug may be cut and bonded in place. If balsa is used, cut the plug so the grain is perpendicular to the skin. If honeycomb material is used, it should be the same density as the original.
Transparent Plastic Repair When a windshield on an unpressurised aircraft has a crack, it can be repaired by: Stop-drill the ends of the crack to prevent it from growing Drill a series of holes ½” from the crack edges, and about ½” apart Lace the holes with brass safety wire to hold the crack together
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Another temporary repair for a cracked windshield is: Stop-drill the ends of the crack Drill holes every inch throughout the crack Install AN515-6 screws with AN365-632 nuts and AN 960-6 washers
Transparent Plastic Materials Storage and Handling Store in a cool dry location away from paint and solvent fumes. Keep protective paper/tape on.
Do not store in direct sunlight (protective paper may deteriorate and become difficult to remove). Store in racks at a 10° angle from vertical. If storing horizontally is unavoidable:
Ensure there is no dirt between sheets,
Stack smallest sheets on top to prevent unsupported overhang, and
Do not stack higher than 18”.
When storing formed sections:
Use ample support to retain shape,
Avoid vertical nesting,
Protect against high temperatures, and
Leave protective coatings on.
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SEALANTS Sealants are manufactured to fulfil a multitude of applications and are generally available in two forms: One part sealants are prepared by the manufacturer and are ready for application as packaged. Two-part sealants are compounds requiring separate packaging to prevent curing prior to application. The two parts are identified as the base sealing compound and the accelerator. Two part sealants are generally mixed by combining equal portions (by weight) of the base and the accelerator compounds and any deviation from the prescribed ratios can reduce the material’s quality.
Some of the applications include sealing fuel tanks and pressurised structure, weatherproofing skin joins and aerodynamic smoothing of aircraft surfaces as well as gluing skins and structure together and separating skins, structure and fasteners as a corrosion preventative.
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TOPIC 6.4.1 AND 6.4.2: CORROSION FUNDAMENTALS AND IDENTIFICATION CORROSION Corrosion is a complex electro-chemical action that causes metals to be transformed into their salts and oxides. These powdery substances replace the metal and cause severe loss of strength in the structure. While complex in its nature, the actual mechanics of corrosion are relatively simple and straight forward.
For corrosion to form, three requirements must be met: An electrical potential difference within the metal A conductive path between the two areas of potential difference Some form of electrolyte or fluid covering the two areas
Corrosion is a natural process, and its prevention almost impossible; but it can be controlled. The aviation technician must prevent or remove one or more of the requirements for corrosion. In doing this goes a long way in adding longevity to the structure of the airplane.
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Cleanliness of the surface is one of the best ways to control corrosion. When moisture is held in contact with the metal surface by an accumulation of dirt or grease, corrosion is sure to start. If the surface can be kept perfectly dry and clean, however, corrosion has little chance of getting started. The essence of corrosion control is therefore prevention rather than removal.
Dry Corrosion At room temperature, most metals carry a thin oxide layer as a result of the reaction of metals with oxygen in the atmosphere. Increase of temperature may cause formation of a heavier layer, or the layer may detach. Zinc and zinc coatings carry a fairly protective zinc hydroxide or carbonate layer (zinc patina) which increases in thickness very slowly. Aluminium carries a thin, highly protective oxide layer. Some corrosion takes place even under completely dry conditions.
Wet Corrosion Wet corrosion takes place in environments where the relative humidity exceeds 60%. The corrosion may be uniform destruction of the metal surface or localised destruction (pitting, stress corrosion cracking). The corrosion can be concentrated adjacent to a more noble metal or at points where the oxygen supply is limited. Wet corrosion is an electro-chemical phenomenon. When two metals are in contact with water solution containing salts, an electric potential is formed between two different metals or the surfaces of the same metal with different surface conditions. This causes the dissolution of the less noble metal. The more noble metal remains protected but the less noble metal corrodes. Issue C: August 2009
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Wet corrosion is most efficient in waters containing salts, such as NaCl (e.g. marine conditions), due to the high conductivity of the solution. Chlorides also may increase the corrosion rate of metals.
Direct Chemical Attack If an alkaline or acidic liquid comes into contact with metal, the result is a form of corrosion known as Direct Chemical Attack. It is also fundamentally electrochemical in nature. However, no current flow is detectable, nor are there any definite anodic or cathodic areas. The theoretical rate of a chemical attack can be affected by the formation of a protective film on the metal surface, through secondary reactions involving the products of corrosion, and the mechanical removal of protective films, such as by erosion, flexing of the metal surface, or by temperature changes. For corrosion to form on a metal there must be an electrode potential difference and an electrolyte. Almost all acids and alkalis react with metals to form metallic salts (corrosion), though some are more active than others. Sulphuric acid as found in batteries is especially active in corroding aluminium, while a weak solution of chromic or phosphoric acid is actually used as a surface treatment when preparing a metal for painting. Ferrous metals are subject to damage from both acids and alkalis, but aluminium is more vulnerable to strong alkaline solutions than to acids. Aluminium structure, for instance, can be severely corroded if allowed to remain on a concrete floor. Water will leach out enough lime from the cement to form an alkaline solution that will corrode the aluminium. Phosphate Ester hydraulic fluids (Skydrol) can cause severs corrosion and embrittlement of titanium alloys at elevated temperatures above 120 °C, probably because of decomposition of the fluid to phosphoric acid.
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Mercury Although it is not commonly found in any quantity around aircraft, there is a definite possibility that mercury could be spilled in an aeroplane. Mercury attacks aluminium by a chemical reaction known as amalgamation. In this process, the mercury rapidly attacks along the grain boundaries of the aluminium, and in an exceedingly short time will completely destroy it. Extreme care must be exercised when removing spilled mercury, as it is “slippery” and will flow through even a tiny crack to get to the lowest part of the structure where it can cause extensive damage. Not only is mercury damaging to aircraft structure, mercury and mercury vapours are also dangerous to people. If mercury is spilled, remove every particle with a vacuum cleaner having a mercury trap in the suction line, or with a rubber suction bulb or medicine dropper. Never attempt to remove mercury by blowing with compressed air. This will scatter It and spread the damage. Brass control cable turnbuckle barrels are especially susceptible to mercury damage. Any sign of mercury discoloration requires replacement of the barrel (brass takes on a slippery, chromed appearance). Often Mercury spills originate in cargo compartments where, rough handling of a passenger baggage has broken a Mercury thermometer carried in the bag. Mercury and Mercury thermometers cannot be legally carried on a commercial aircraft
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Electro-Chemical Series If the anode is smaller than the cathode, the anode will give up electrons more easily and the corrosion will be more rapid.
Galvanic Corrosion Galvanic Corrosion can take place where dissimilar metal skins are riveted together, or where aluminium inspection plates are attached to structure with steel screws.
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Stress Corrosion Stresses may come from a fitting that has been pressed into a structural part with an interference fit. Cracks grow rapidly as the corrosive attack concentrates at the end of the crack, rather than along its sides as in intergranular corrosion. A common place for stress corrosion is between rivets in a stressed skin, around pressed-in bushing, tapered pipe fittings. Careful visual inspection may show up this type of corrosion, but to find the actual extent of the cracks requires dye penetrant inspection.
Surface Corrosion Where an area of unprotected metal is exposed to an atmosphere containing battery fumes, exhaust gases, or industrial contaminants, there will be a rather uniform attack over the entire surface area. This dulling of the surface is caused by microscopic amounts of the metal being converted into corrosion salts. If these deposits are not removed and the surface protected against further action, there will develop such a rough surface that corrosion pits will form. Surface corrosion can go undetected until it breaks through the metal, when it is too late to save the affected parts.
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Pitting Corrosion A logical progression from uniform surface corrosion, if left untreated, is called pitting. Pits form as localized anodic areas. Corrosive action may continue until an appreciable percentage of the metal thickness is converted into salts which may. In extreme cases, eat completely through the metal. Pitting corrosion may be detected by the appearance of clumps of white powder on the surface.
Intergranular Corrosion Micro-photographs of aluminium alloys show them made up of extremely tiny grains held together by chemical bonds; that is, the interaction of the atoms of the various elements. In the process of heat treating, the metal is heated to such a temperature that these alloying agents go into solution with each other. When this temperature has been reached uniformly throughout, the metal is removed from the furnace and immediately quenched in water to solidify these elements into extremely small grains. If quenching is delayed, for even a few seconds, these grains will grow, and when finally quenched, will have reached such size that the areas of dissimilar metals will provide efficient cathodes and anodes for corrosion formation. It is hard to detect as it is inside the metal but often shows up as a blister on the surface.
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Filiform Corrosion Filiform corrosion is a particular case of oxygen cell corrosion in a protective organic coating. In certain conditions corrosion can be initiated at the defect and can propagate beneath the organic coating: the front of the propagation acting as the anode (low aerated zone) and the defect neighbourhood acting as the cathode (high aerated zone). The unusual characteristic of this type of corrosion is that it forms in the areas where there is a deficiency of oxygen. A wash primer is a two-art metal preparation material in which phosphoric acid converts the surface of the metal in to a phosphate film that protects the metal from corrosion, and provides an excellent bond for paint. The conversion process relies on moisture in the air, and if there is not enough moisture to convert all of the acid, some acid remains on the metal. If a dense polyurethane finish is then applied, the acid becomes trapped and reacts with the aluminium alloy to form corrosion.
Fretting Corrosion When two surfaces fit tightly together, but can move relative to one another, they may be eroded. These surfaces are normally not close enough together to shut out oxygen so they do develop the desired protective film. However, this film is destroyed by the continued rubbing action. This wear is fretting corrosion. When movement between the two surfaces is small, the debris between them does not have an opportunity to escape, and it acts as an abrasive to further erode the surfaces. By the time this type of corrosion makes its appearance on the surface, the damage is usually done and the parts must be replaced. Fretting corrosion may occur around rivets in a skin. This will be indicated by dark deposits around the rivet heads, streaming out behind. It gives the appearance of the rivet smoking. Rivets showing this sign of fretting must be replaced as soon as possible.
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Exfoliation Corrosion This type of corrosion, despite its high sounding name, is simply an extreme case of intergranular corrosion. It occurs chiefly in extruded materials such as channels or angles where the grain structure Is more laminar (layer-like) than In rolled sheets or castings. This type of corrosion occurs along the grain boundaries and causes the material to separate or delaminate. As with other types of Inter- granular corrosion, by the time it is evident on the surface, the strength of the metal has been greatly decreased. In this state the metal no longer has any strength. The only remedy is complete removal of the affected sections.
Microbiological Growths New developments bring new problems. For years, water which condensed in the fuel tanks produced relatively minor corrosion problems. Small perforated metal containers of potassium dichromate crystals protected the fuel tanks by changing any water into a mild chromic acid solution which inhibited corrosion. Jet aircraft, however, use fuel with a higher viscosity than gasoline, fuel which holds more water in suspension. They also fly higher than reciprocating engine aircraft. In high altitude, low temperature flight conditions, the water entrained in the fuel will condense out and collect in the bottom of the tanks.
To further complicate matters, this water contains microbes which are simply microscopic sized animal life and plant life. These organic bodies live in the water and feed on the hydrocarbon fuel. The dark Insides of the fuel tanks promote their growth and before you know it, these tiny creatures Issue C: August 2009
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have multiplied until they form a scum in the tank. It holds water in contact with the tank structure where corrosion of the concentration cell type will inevitably form. If the scum forms along the edge of a seal in an integral fuel tank, the sealant may pull away from the structure, causing a leak and an expensive resealing operation. Figure below shows the lower skin of a fuel tank - the splotchy areas are fungus, the darker area around the corner of the tank is sealant.
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TOPIC 6.5.4: AIRCRAFT RIVETS RIVETS SPECIFICATION AND STANDARDS Specifications and standards for aircraft hardware are generally identified by the organization originating them. Some of the most common are: AMS - Aeronautical Material Specifications AN - Air Force-Navy AND - Air Force-Navy Design AS - Aeronautical Standard ASA - American Standards Association American Society for Testing and Materials Military Standard Naval Aircraft Factory National Aerospace Standard Society of Automotive Engineers A rivet is any type of fastener that obtains its clamping action by having one of its ends mechanically upset. When an MS20470-AD4-4 rivet is required, specifications have already been written for it and are available to both the aircraft manufacturer and rivet producer. These specifications stipulate the material to be used as well as the rivet dimensions. By using these specifications and calling for standard hardware, aircraft manufacturers are able to build reproducible aircraft at an economical cost.
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Solid Shank Rivets The solid shank rivet has been used since sheet metal was first utilized in aircraft, and remains the single most commonly used aircraft fastener today. Unlike other types of fasteners, rivets change in dimension to fit the size of a hole. When a rivet is driven, it’s cross sectional area increases along with its bearing and shearing strengths. Solid shank rivets are available in a variety of materials, head designs, and sizes to accommodate different applications. Before driven, a rivet should extend beyond the base material at least 1.5 times the rivet’s diameter. Once driven, the rivet shank expands to fill the hole and the bucktail expands to 1.5 times its original diameter. Once the bucktail expands to the appropriate diameter it should extend beyond the base material by at least one half the original rivets diameter.
Rivet Codes Rivets are given part codes that indicate their size, head style, and alloy material. Two systems are in use today, the Air Force - Navy, or AN system, and the Military Standards 20 system, or MS2O. While there are minor differences between the two systems, both use the same method for describing rivets.
The first part denotes numbering system. Second part describes head style (For examples, 470 is universal head and 426 is countersunk). Third part is a one or two digit letter code representing rivet alloy material. Next is the shank diameter indicated in 1/32 inch increments. Issue C: August 2009
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Finally the length indicated in 1/16 inch increments.
The length of a universal head (AN470) rivet is measured from the bottom of the manufactured head to the end of the shank. However, the length of a countersunk rivet (AN426) is measured from the top of the manufactured head to the end of the shank.
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Rivets Head Design As mentioned, solid shank rivets are available in two standard head styles, universal and countersunk, or flush. The AN470 universal head rivet now replaces all previous protruding head styles such as AN430 round, AN442 flat, AN455 brazier, and AN456 modified brazier. Joints utilizing countersunk rivets generally lack the strength of protruding head rivet joints. One reason is that a portion of the material being riveted is cut away to allow for the countersunk head., Another reason is that, when riveted, the gusset may not make direct contact with the rivet head if the rivet hole was not countersunk or dimpled correctly.
AN426 countersunk rivets were developed to streamline airfoils and permit a smooth flow over an aircraft’s wings or control surfaces. However, before a countersunk rivet can be installed, the metal must be countersunk or dimpled. Countersinking is a process in which the metal in the top sheet is cut away in the shape of the rivet head. On the other hand, dimpling is a process that mechanically “dents” the sheets being joined to accommodate the rivet head. Sheet thickness and rivet size determine which method is best suited for a particular application.
Joints utilizing countersunk rivets generally lack the strength of protruding head rivet joints. One reason is that a portion of the material being riveted is cut away to allow for the countersunk head., Another reason is that, when riveted, the gusset may not make direct contact with the rivet head if the rivet hole was not countersunk or dimpled correctly, dimpled correctly, resulting in the rivet not expanding to fill the entire hole. To ensure head-to-gunset contact, it is recommended that countersunk heads be installed with the manufactured head protruding above the skin’s surface about .005 to .007 of an inch. This ensures that the gunset makes direct contact with the rivet head.
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To provide a smooth finish after the rivet is driven, the protruding rivet head is removed using a Microshaver. This rotary cutter shaves the rivet head flush with the skin, leaving an aerodynamically clean surface. Figure A - if the rivet head is allowed to protrude above the metal all of the gunset’s energy hits the head resulting in a stronger joint. Figure B - if a countersunk rivet is set with the rivet head flush with the metal’s surface, some of the gunset’s driving energy is lost.
Rivet Alloys Most aircraft rivets are made of aluminium alloy. The type of alloy is identified by a letter in the rivet code and by a mark on the rivet head itself.
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1100 Aluminium (A) Rivets made of pure aluminium have no identifying marks on their manufactured head, and are designated by the letter A in the rivet code. Since this type of rivet is made out of commercially pure aluminium, the rivet lacks sufficient strength for structural applications. Instead, 1100 rivets are restricted to non-structural assemblies such as fairings, engine baffles, and furnishings. The 1100 rivet is driven cold, and therefore, its shear strength increases slightly as a result of cold working.
2117 Aluminium Alloy (AD) The rivet alloy 2117-T3 is the most widely used for manufacturing and maintenance of modern aircraft. Rivets made of this alloy have a dimple in the centre of the head and are represented by the letters put in rivet part codes. Because AD rivets are so common and require no heat treatment, they are often referred to as “field rivets.” The main advantage for using 2117-T3 for rivets is its high strength and shock resistance characteristics. The alloy 2117-T3 is classified as a heat-treated aluminium alloy, but does not require re-heat treatment before driving.
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5056 Aluminium Alloy (B) Some aircraft parts are made of magnesium. If aluminium rivets were used on these parts, dissimilar metal corrosion could result. For this reason, magnesium structures are riveted with 5056 rivets which contain about 5 percent magnesium. These rivets are identified by a raised cross on their heads and the letter B in a rivet code. The maximum shear strength of an installed 5056H32 rivet is 28,000 pounds per square inch.
2017 Aluminium Alloy (D) 2017 aluminium alloy is extremely hard. Rivets made of this alloy are often referred to as D rivets and were widely used for aircraft construction for many years. However, the introduction of jet engines placed greater demands for structural strength on aircraft materials and fasteners. In response to this, the aluminium industry modified 2017 alloy to produce a new version of 2017 aluminium, called the crack free rivet alloy. The minimum shear strength of the older 2017T31 rivet alloy is 30 KSI, while that of the new 2017T3 alloy is 34 KSI (KSI = Kilo pounds per inch).
D-rivets are identified by a raised dot in the centre of their head and the letter D in rivet codes. Because D-rivets are so hard they must be heat treated before they can be used. When aluminium alloy is quenched after heat treatment it does not harden immediately. Instead, it remains soft for several hours and gradually becomes hard and gains full strength. Rivets made of 2017 can be kept in this annealed condition by removing them from a quench bath and immediately
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storing them in a freezer. Because of this, D rivets are often referred to as icebox rivets. These rivets become hard when they warm up to room temperature, and may be reheat-treated as many times as necessary without impairing their strength. 2024 Aluminium Alloy (DD) DD-rivets are identified by two raised dashes on their head. Like D-rivets, DD-rivets are also called icebox rivets and must be stored at cool temperatures until they are ready to be driven, The length of time the rivets remain soft enough to drive is determined by the storage temperature. For example, if the storage temperature is -30 degree F, the rivets will remain soft enough to drive for two weeks. When DD rivets are driven their alloy designation becomes 2024T31.
7050-T73 Aluminium Alloy (E) A new and stronger rivet alloy was developed in 1079 called 7050T73. The letter E is used to designate this alloy, and the rivet head is marked with a raised circle. 7050 alloy contains zinc as the major alloying ingredient and is precipitation heat-treated. This alloy is used by the Boeing Airplane Company as a replacement for 2024T31 rivets in the manufacture of the 767 wide-body aircraft.
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Corrosion-Resistant Steel (F) Stainless steel rivets are used for fastening corrosion-resistant steel sheets in applications such as firewalls and exhaust shrouds. They have no marking on their heads and designated with the letter ‘F’.
Monel (M) Monel rivets are identified with two recessed dimples in their heads. They are used in place of corrosion-resistant steel rivets when their somewhat lower shear strength is not a detriment.
Rivet Heat Treatment When an alloy is allowed to cool at room temperature, it is referred to as natural aging and can take several hours or several weeks. This process can be accelerated by carrying out “precipitation heat-treatment”, also known as artificial aging and precipitation-hardening. This process applies a slightly elevated temperature for an extended period to attain the required temper in the shortest time possible.
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Special Rivets Conventional solid shank rivets require access to both ends to be driven. However, special rivets, often called blind rivets are installed with access to only one end of the rivet. While considerably more expensive than solid shank rivets, blind rivets find many applications in today’s aircraft industry. Pop™ Rivets Pop rivets have limited use on aircraft and are never used for structural repairs. However, they are useful for temporarily lining up holes. In addition, some “home built” aircraft utilize Pop rivets. They are available in flat head, countersunk head, and modified flush heads with standard diameters of 1/8, 5/32, and 3/16 inch. Pop rivets are made from soft aluminium alloy, steel, copper, and Monel.
Friction-Lock Rivets One early form of blind rivet that was the first to be widely used for aircraft construction and repair was the Cherry friction-lock rivet. Originally, Cherry friction-locks were available in two styles, hollow shank pull-through and self plugging types. The pull-through type is no longer common; however, the self-plugging Cherry friction-lock rivet is still used for repairing light aircraft. Cherry friction-lock rivets are available in two head styles, universal and 100 degree countersunk. Furthermore, they are usually supplied in three standard diameters, 1/8, 5/32, and 3/16 inch. However, larger sizes can be specially ordered in sizes up to 5/16 inch. The friction-lock rivet assembly consists of a shell and mandrel or pulling stem. The stem is pulled until the header forms a bucktail on the blind side of the shell. At this point, a weak point built into the stem shears and the stem breaks off. After the stem fractures, part of it projects upward. The projecting stem is cut close to the rivet head and the small residual portion is filed smooth.
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A friction-lock rivet cannot replace a solid shank rivet, size for size. When a friction-lock is used to replace a solid shank rivet, it must be at least one size (1/32 inch) larger in diameter. This is because a friction-lock rivet loses considerable strength if its centre stem falls out due to damage or vibration.
Mechanical-Lock Rivets Mechanical-lock rivets were designed to prevent the centre stem of a rivet from falling out as a result of the vibration encountered during aircraft operation. Unlike the centre stem of a friction-lock rivet, a mechanical-lock rivet permanently locks the stem into place and vibration cannot shake it loose. Mechanical-lock rivets include: Huck-Loks Cherrylocks Olympic-Loks Cherrymax
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Huck-Loks Huck-Lok rivets were the first mechanical-lock rivets and are used as structural replacements for solid shank rivets. However, because of the expensive tooling required for their installation, HuckLoks are generally limited to aircraft manufacturers and some large repair facilities. Huck-Loks are available in four standard diameters,1/8, 5/32, 31/6, and 1/4 inch, and come in three different alloy combinations: a 5055 sleeve with a 2024 pin, an A-286 sleeve with an A-285 pin, and a Monel 400 sleeve with an A-285 pin.
Cherrylocks™ The Cherry mechanical-lock rivet, often called the bulbed CherryLOCK, was developed shortly after the Huck-Lok. Like the Huck-Lok, the CherryLOCK rivet is an improvement over the frictionlock rivet because its center stem is locked into place with a lock ring. This results in shear and bearing strengths that are high enough to allow CherryLOCKS to be used as replacements for solid shank rivets. CherryLOCK rivets are available with two head styles, 100 degree countersunk and universal. Like most blind rivets, CherryLOCKs are available with diameters of 1/8, 5/32, and 3/16 inch, with an oversize of 1/64 inch for each standard size. The rivet, or shell, portion of a CherryLOCK may be constructed of 2017 aluminium alloy, 5056 aluminium alloy, Monel, or stainless steel. Installation of CherryLOCK rivets requires a special pulling tool for each different size and head shape. However, the same size tool can be used for an oversize rivet in the same diameter group.
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One disadvantage of a CherryLOCIC is that if a rivet is too short for an application, the lock ring sets pre maturely resulting in a malformed shank header. This fails to compress the joint, leaving it in a weakened condition. To avoid this, always use the proper rivet length selection gauge and follow the manufacturer’s installation recommendations.
Olympic-LOKS Olympic-LOK blind fasteners are light weight, mechanically-locking spindle-type blind rivets. Olympic-loks come with a lock ring stowed on the head. As an Olympic-lok is installed, the ring slips down the stem and locks the centre stem to the outer shell. These blind fasteners require a specially designed set of installation tools. Olympic-lok rivets are made with three head styles: universal, 100 degree flush, and 100 degree flush shear. Rivet diameters of 1/8, 5/32, and 3/16 inch are available in eight different alloy combinations of 2017-T4, A-286, 5056, and Monel. The installation tools fit both countersunk and universal heads in the same size range.
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CHERRYMAX™ The CherryMAX rivet is economical to use and strong enough to replace solid shank rivets, size for size. The economic advantage of the CherryMAX system is that one size puller can be used for the installation of all sizes of CherryMAX rivets. A CherryMAX rivet is composed of five main parts: a pulling stem, a driving anvil, a safe-lock locking collar, a rivet sleeve, and a bulbed blind head. Available in both universal and countersunk head styles, the rivet sleeve is made from 5056, monel, and inco 600. The stems are made from alloy steel, CRES, and inco X-750. The ultimate shear strength of CherryMAX rivets ranges from 5OKSI to 75KS1, Furthermore, CherryMAX rivets can be used at temperatures from 1200 C to 7600 C. They are available in diameters of 1/8, 5/32, 3/16 and 1/4 inches and are also made with an oversize diameter for each standard diameter listed.
Removal Of Mechanical-Lock Rivets To remove mechanical-lock rivets, you must first file a flat spot on the rivet’s centre stem. Once this is done, a centre punch is used to punch out the stem so the lock ring can be drilled out. With the lock ring removed you can tap out the remaining stem, drill to the depth of the manufactured head, and tap out the remaining shank. All brands of mechanical-lock blind rivets are removed using the same basic technique.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Hi-Shear Rivets One of the first special fasteners used by the aero-space industry was the Hi-Shear rivet. Hi-Shear rivets were developed in the 1940s to meet the demand for fasteners which could carry greater shear loads. The Hi-shear rivet has the same strength characteristics as a standard AN bolt. In fact, the only difference between the two is that a bolt is secured by a nut and a Hi-Shear rivet is secured by a crushed collar. The Hi-Shear rivet is installed with an interference fit, where the side wall clearance is reamed to a tolerance determined by the aircraft builder. When properly installed, a Hi-Shear rivet has to be tapped into its hole before the locking collar is swaged on. Hi-Shear rivets are made in two head styles, fiat and countersunk. As the name implies, the HiShear rivet is designed especially to absorb high shear loads. The Hi-Shear rivet is made from steel alloy having the same tensile strength as an equal size AN bolt. The lower portion of its shank has a specially milled groove with a sharp edge that retains and finishes the collar as it is swaged into the locked position.
Special Fasteners Many special fasteners have the advantage of producing high strength with light weight and can be used in place of conventional AN bolts and nuts. When a standard AN nut and bolt assembly is tightened, the bolt stretches and its shank diameter decreases, causing the bolt to increase its clearance in the hole. Special fasteners eliminate this change in dimension because they are held in place by a collar that is squeezed into position instead of being screwed on like a nut. As a result, these fasteners are not under the same tensile loads imposed on a bolt during installation. Lockbolts Lockbolts are manufactured by several companies and conform to Military Standards. These standards describe the size of a lockbolt’s head in relation to its shank diameter, as well as the alloy used. Lockbolts are used to assemble two materials permanently. They are lightweight and are as strong as standard bolts. There are three types of lockbolts used in aviation, they are: Pull-type lockbolt Blind-type lockbolt Stump-type lockbolt
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
The pull-type lockbolt has a pulling stem on which a pneumatic installation gun fits. The gun pulls the materials together and then drives a locking collar into the grooves of the lockbolt. Once secure, the gun fractures the pulling pin at its break point. The blind-type lockbolt is similar to most other types of blind fasteners. To install a blind lockbolt, it is placed into a blind hole and an installation gun is placed over the pulling stem. As the gun pulls the stem, a blind head forms and pulls the materials together. Once the materials are pulled tightly together, a locking collar locks the bolt in place and the pulling stem is broken off. Unlike other blind fasteners that typically break off flush with the surface, blind lockbolts protrude above the surface. The third type of lockbolt is the stump-type lockbolt and is installed in places where there is not enough room to use the standard pulling tool. Instead, the stump-type lockbolt is installed using an installation tool similar to that used to install Hi-Shear rivets.
Lockbolts are available for both shear and tension applications. With shear lockbolts, the head is kept thin and there are only two grooves provided for the locking collar. However, with tension lockbolts, the head is thicker and four or five grooves are provided to allow for higher tension values. The locking collars used on both shear and tension lockbolts are colour coded for easy identification.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Hi-Loks Hi-Lok bolts are manufactured in several different alloys such as titanium, stainless steel, steel, and aluminium. They possess sufficient strength to with stand bearing and shearing loads, and are available with flat and countersunk heads. A conventional Hi-Lok has a straight shank with standard threads. Although wrenching lock nuts are usually used, the threads are compatible with standard AN bolts and nuts. To install a Hi-Lok, the hole is first drilled with an interference fit. The Hi-Lok is then tapped into the hole and a shear collar is installed. A Hi-Lok retaining collar is installed using either specially prepared tools or a simple Allen and box end wrench. Once the collar is tightened to the appropriate torque value, the wrenching device shears off leaving only the locking collar.
Hi-Lites The Hi-Lite fastener is similar to the Hi-Lok except that it is made from lighter materials and has a shorter transition from the threaded section to the shank. Furthermore, the elimination of material between the threads and shank give an additional weight saving with no loss of strength. The HiLite’s main advantage is its excellent strength to weight ratio. Hi-Lites are available in an assortment of diameters ranging from 3/16 to 3/8 inch. They are installed either with a Hi-Lok locking collar or by a swaged collar like the Lockbolt. In either case, the shank diameter is not reduced by stretch torquing. Cherrybucks The CherryBUCK is a one-piece special fastener that combines two titanium alloys which are bonded together to form a strong structural fastener. The head and upper part of the shank of a CherryBUCK is composed of 6AL-4V alloy while Ti-Cb alloy is used in the lower shank. When driven, the lower part of the shank forms a bucktail.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
An important advantage of the CherryBUCK is the fact that it is a one piece fasteners. Since there is only one piece, CherryBUCKs can safely be installed in jet engine intakes with no danger of foreign object damage. This type of damage often occurs when multiple piece fasteners lose their retaining collars and are ingested into a compressor inlet.
Taper-Lok Taper-Loks are the strongest special fasteners used in aircraft construction. Because of its tapered shape, the Taper-Lok exerts a force on the conical walls of a hole, much like a cork in a bottle. To a certain extent, a Taper-Lok mimics the action of a driven solid shank rivet, in that it completely fills the hole. However, a Taper-Lok does this without the shank swelling. When a washer nut draws the Taper-Lok into its hole, the fastener pushes outward and creates a tremendous force against the tapered walls of the hole. This creates radial compression around the shank and vertical compression lines as the metals are squeezed together. The combinations of these forces generate strength unequalled by any other type of fastener.
Hi-Tigue The Hi-Tigue fastener has a bead that encircles the bottom of its shank and is a further advancement in special fastener design. This bead preloads the hole it fills, resulting in increased joint strength. During installation, the bead presses against the side wall of the hole, exerting a radial force which strengthens the surrounding area. Since it is preloaded, the joint is not subjected to the constant cyclic action that normally causes a joint to become cold worked and eventually fail. Hi-Tigue fasteners are produced in aluminium, titanium, and stainless steel alloys. The collars are also composed of compatible metal alloys and are available in two types, sealing and non-sealing.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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As with Hi-Loks, Hi-Tigues can be installed using an Allen and box end wrench.
Jo-Bolts These patented high-strength structural fasteners are used in close-tolerance holes where strength requirements are high but physical clearance precludes the use of standard AN, MS, or NAS bolts. The hole for a Jo-Bolt is drilled, reamed, and countersunk before the Jo-Bolt is inserted and held tightly in place by a nose adapter of either a hand tool or power tool. A wrench adapter then grips the bolt’s driving flat and screws it up through the nut. As the bolt pulls up it forces a sleeve up over the tapered outside of the nut and forms a blind head on the inside of the work. When driving is complete, the driving flat of the bolt breaks off.
Removal Of Special Fasteners Special fasteners that are locked into place with a crushable collar are easily removed by splitting the collar with a small cape chisel. After the collar is split, knock away the two halves and tap the fastener from the hole. Fasteners which are not damaged during removal can be reused using new locking collars. The removal techniques of certain special fasteners are basically the same as those used for solid shank rivets. However, in some cases, the manufacturer may recommend that a special tool be used. Removal of Taper-Loks, Hi-Loks, Hi-Tigues, and Hi-Lites requires the removal of the washer nut or locking collar. Both are removed by turning them with the proper size box end wrench or a pair of vice-grips. After removal, a mallet is used to tap the remaining fastener out of its hole.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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To remove a Jo-Bolt, begin by drilling through the nut head with a pilot bit followed by a bit of the same size as the bolt shank. Once the nut head is removed, a punch is used to punch out the remaining portion of the nut and bolt.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
TOPIC 6.6.1 & 6.6.2: PIPES AND UNIONS RIGID LINES A single aircraft typically contains several different types of rigid fluid lines. Each type of line has a specific application. However, as a rule, rigid tubing is used in stationary applications and where long, relatively straight runs are possible. Systems that typically utilize rigid tubing include fuel, oil, oxygen, and instrument systems. Many fluid lines used in early aircraft were made of copper tubing. However, copper tubing proved trouble some because it became hard and brittle from the vibration encountered during flight, and eventually failed. To help prevent failures and extend the life of copper tubing, it must be periodically annealed to restore it to a soft condition. Annealing is accomplished by heating the tube until it is red hot and then quenching in cold water. When working on an aircraft that has copper tubing, the tubing should be annealed each time it is removed. Furthermore, copper lines must be regularly inspected for cracks, hardness, and general condition.
Today, aluminium-alloy and corrosion-resistant steel lines have replaced copper in most applications. Aluminium tubing comes in a variety of alloys. For example, in low pressure systems (below 1,000 psi) such as those used for instrument air or ventilating air, commercially pure aluminium tubing made from 1100- H14 (half-hard), or aluminium alloy 3003-H14 (half-hard) is used. Low pressure fuel and oil and medium pressure (1,000 to 1,500 psi) hydraulic and pneumatic systems often use lines made of 5052-0 aluminium alloy. This alloy, even in its annealed state, is about one and three-quarters times stronger than half-hard, commercially pure aluminium. Occasionally, 2024-T aluminium alloy is used for fluid lines because of its higher strength. However, it is not as flexible and, therefore, is more difficult to bend and flare without cracking.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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Aluminium alloy tubes are identified in a number of ways. For example, on larger tubes, the alloy designation is stamped directly on the tube’s surface, however, on small tubing; the alloy designation is typically identified by a coloured band. These colour bands are no more than 4 inches wide and are painted on the tube’s ends and mid section. When a band consists of two colours, one-half the width is used for each colour.
Corrosion-resistant steel tubing, either annealed or 1/4 hard, is used in high pressure systems (3,000 psi). Applications include high pressure hydraulic, pneumatic and oxygen systems. Corrosion-resistant steel is also used in areas that are subject to physical damage from dirt, debris, and corrosion caused by moisture, exhaust fumes, and salt air. Such areas include flap wells and external brake lines. Another benefit of corrosion-resistant steel tubing is that it has a higher tensile strength which permits the use of tubing with thinner walls. As a result, the installation weight is similar to that of thicker-walled aluminium alloy tubing.
Size Designations The size of rigid tubing is determined by its outside diameter in increments of 1/16 inch. Therefore, a -4 ‘B’ nut tubing is 4/16 or 1/4 inch in diameter. A tube diameter is typically printed on all rigid tubing. Another important size designation is wall thickness, since this determines a tube’s strength. Like the outside diameter, wall thickness is generally printed on the tube in thousandths of an inch. One dimension that is not printed on rigid tubing is the inside diameter. However, since the outside diameter and wall thickness are indicated, the inside diameter is determined by subtracting twice the wall thickness from the outside diameter. For example, if you have a piece of -8 tubing with a wall thickness of 0.072 inches, you know the inside diameter is .356 inches, 0.5 — (2 x .072) = 0.356.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Tube Flaring Much of the rigid tubing used in modem aircraft is connected to components by flaring the tube ends and using flare-type fittings. A flared-tube fitting consists of a sleeve and a B-nut. Using this type connector eliminates damage to the flare caused by the wiping or ironing action as the nut is tightened. The sleeve provides added strength and supports the tube so that vibration does not concentrate at the flare. The nut fits over the sleeve and, when tightened, draws the sleeve and flare tightly against a male fitting to form a seal. The close fit between the inside of the flared tube and the flare cone of the male fitting provides the actual seal. Therefore, these two surfaces must be absolutely clean and free of cracks, nicks, and scratches. Aircraft fittings have a flare angle of 37 degrees and are not interchange able with automotive-type fittings, which have a flare angle of 45 degrees. The combination of an AN 818 nut and AN 819 sleeve provides a tight, leak-free attachment that does not damage the flare.
There are two types of flares used in aircraft plumbing systems, the single flare and the double flare. As discussed, the flare provides the sealing surface and is subject to extremely high pressures. Because of this, flares must be properly formed to prevent leaks or failures. A flare which is made too small produces a weak joint, and may leak or pull apart. On the other hand, if a flare is too large it may interfere with the installation of the nut and result in leakage. In either case, if a fitting leaks when properly torqued, you should inspect the flare and fitting components for proper manufacture and assembly. Do not over-tighten a leaky fitting.
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Part-66 Subject
Flared Tube Fittings Flared fittings are identified by either an AN or MS number. However, prior to World War II fittings were made to an AC standard. Since AC fittings are still used in some older aircraft, it is important that you be able to identify the differences in fittings. For example, an AN fitting has a shoulder between the end of the threads and the flare cone. The AC fitting does not have this shoulder. Another difference between AC and AN fittings includes the sleeve design. The AN sleeve is noticeably longer than the AC sleeve of the same size.
Flared-tube fittings are made of aluminium alloy, steel, or copper base alloys. For identification purposes, all AN steel fittings are coloured black, and all AN aluminium fittings are coloured blue. The AN 819 aluminium bronze sleeves are cadmium plated and are not coloured. AN fittings come in a variety of shapes and sizes, each with a specific use. As an air craft technician, you must be familiar with the most common fittings used on aircraft.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Universal Bulkhead Fitting One specific type of fitting is the universal bulkhead fitting. As the name implies, a bulkhead fitting is used to support a line that passes through a bulkhead. Bulkhead fittings have straight machine threads, similar to those on common nuts and bolts. Therefore, flared tube connections, crush washers, or synthetic seals must be used to make these connections fluid-tight.
Tapered Pipe Thread Fittings Fluid lines are commonly attached to components by tapered pipe thread fittings. Tapered pipe thread fittings create a seal by wedging the tapered external male thread and the tapered internal female threads. This is the same type of thread used in household plumbing and automotive applications. These threads taper 1/16 inch to the inch, When working with these fittings, care must be exercised when screwing them into cast aluminium or magnesium housings so that the fitting is not screwed tight enough to crack the casting. When joining tapered pipe thread fittings a small amount of thread lubricant or a thin strip of Teflon tape on the male threads ensures that fittings screw together tightly enough to form a complete seal. However, some manufacturers do not recommend Teflon tape because tape fragments can be introduced into a system if the tape is not applied correctly. For this reason you should always follow the manufacturer’s directions.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Flareless Fittings The heavy wall tubing used in some high-pressure systems is difficult to flare. For these applications, the flareless fitting is designed to provide leak-free attachments without flares. Although the use of flareless fittings eliminates the need to flare the tube, a step referred to as presetting is necessary prior to installation of a new flareless tube assembly. Presetting is the process of applying enough pressure to the sleeve to cause it to cut into the outside of the tube.
The tube end is fitted into the union and the nut tightens the ferrule up against it creating a seal.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Banjo Fittings Banjo fittings are available in all standards and used in many engine and airframe fluid systems. In some applications o-rings are used in place of the aluminium gaskets shown. The Banjo is free to rotate before the bolt is torqued; this allows stress free alignment of the fluid line to the component.
Quick Disconnect Couplings Couplings provide quick connect/disconnect capability with self-sealing action for use on Ground Support Equipment (GSE) and other aerospace applications. Simple one hand operation to connect and disconnect with positive visual and touch indicate full engagement. Commonly used on fuel, hydraulic, water/waste servicing adapters. Built in shutoff valve provides a no-spill connection and disconnect. Some have a multi-start thread ¼ turn attachment others have a sliding collar, snap-lock attachment.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Beading Large diameter lines carrying low-pressure fluids such as engine return oil and cooling air are typically joined by a rubber hose that is slipped over the tube ends and held in place with screwtype hose clamps. However, for this to be effective the tube must be beaded first.
This can be accomplished with either a power header or a hand beading tool. The diameter and wall thickness of the tube being beaded determine which is used.
For example, a hand-beading tool is used with tubing having ¼ inch to 1 inch outside diameter. When using a hand-beading tool, the bead is formed by a beader frame with the proper rollers. The sizes, which are marked on the rollers in sixteenths of an inch, correspond with the outside tube diameter. Separate rollers are required for the inside of different sized tubing so care must be taken to use the correct parts when beading. The beading tool operates somewhat like a tube cutter in that the roller is screwed down while rotating the beading tool around the tubing. However, the inside and outside of the tube must be lubricated with a light oil to reduce friction during beading.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
When joining two beaded tubes, begin by slipping the hose over the beads and centering the hose clamps between the ends of the hose and the beads. Next, tighten the clamps finger-tight followed by one and one-half to two complete turns using a wrench or pliers. When doing this, be careful not to over tighten the clamps or you could cause excessive “cold-flow,” which is indicated by deep, permanent impressions in the hose.
Identification of Fluid Lines Large aircraft contain plumbing systems for many different types of fluids. Because of this, it is important that each line be clearly identified. This is generally accomplished by marking tubing with colour bands, symbols, or writing.
The symbols are generally printed on one-inch wide tape or decals and secured at regular intervals along a line. On lines four inches or larger in diameter, or those subject to extreme temperatures, steel tags are used instead of marking tape. In areas where there is the possibility that tape, decals, or tags may be drawn into the induction system, paint is used. Additional markings are sometimes provided to identify a line’s function. These include PRESSURE, RETURN, DRAIN, and VENT. In addition to colour bands, some lines carrying fuel are marked with the word “FLAM.” This identifies the lines as carrying a flammable fluid. Lines carrying fluids that are physically dangerous such as oxygen, nitrogen, or Freon are marked “PHDAN.”
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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Generally, tapes and decals are placed on both ends of a line and at least once in each compartment through which the line runs. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessory within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.
Flexible Fluid Lines Flexible hose is used in aircraft fluid systems to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems.
Flexible hose construction generally consists of an inner liner covered with layers of reinforcement to provide strength and an outer cover to protect from physical damage. The materials and manufacturing process of each layer determine the suitability of a specific hose for a particular application.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
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Inner Liners The inner liner of a flexible hose carries the fluid and, therefore, must have a minimum porosity and be chemically compatible with the material being carried. Furthermore, the liner must be smooth to offer the least resistance to flow, and remain flexible throughout an entire range of operating temperatures. There are basically four different synthetic compounds used in the construction of the inner liner. They are Neoprene, Buna-N, Butyl and Teflon. Each of these compounds has different characteristics and is compatible with different types of fluid. Neoprene is a form of synthetic rubber that is abrasion resistant and is used with petroleum- based fluids. Buna-N is a synthetic rubber compound that is also used to carry petroleum-based products. In fact, Buna-N is better suited to carry petroleum products than neoprene. Butyl is a synthetic rubber compound made from petroleum raw materials and, therefore, breaks down if used with petroleum products. However, butyl is excellent as an inner liner for fluid lines carrying phosphate ester-base hydraulic fluids such as Skydrol®. Teflon is the DuPont trade name for Tetrafluoroethylene resin. Teflon® has an extremely broad operating temperature range (—65°F to 450°F) and is compatible with nearly every liquid used. Furthermore, its unique wax-like surface offers minimum resistance to fluid flow. Because of its unique chemical structure, Teflon experiences less volumetric expansion than rubber and has an almost limitless shelf and service life. When installing Teflon hose, you must always observe minimum bend radius restrictions. Furthermore, after the hose has been in service, it ‘takes a set,’ or becomes somewhat rigid. Therefore, if a hose is removed from the aircraft, it must not be bent against this set. To further reduce the possibility of damaging a hose, certain hose assemblies are manufactured with a preset shape. When this is done, they are usually shipped with a wire holding the assembly in its preset shape. When working with a hose that is shipped this way, the wire should remain in place until the assembly is installed. If a pre-set assembly is removed from an aircraft, a wire should be installed prior to removal to help the hose maintain its shape.
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Reinforcement Layers The reinforcement layers placed over an inner liner determine the strength of a hose. Common reinforcement layers are made of cotton, rayon, polyester fabric, carbon-steel wire, or a stainless steel wire braid Since hose has a tendency to increase in diameter and decrease in length when pressure is applied, the design of the reinforcement is critical. The proper design of the reinforcement layers can minimize these dimensional changes. Outer Cover A protective outer cover, usually made of rubber-impregnated fabric or stainless steel braid, is put over the reinforcement to protect the hose from physical damage. In areas of high heat the outer cover is often designed as an integral fire-sleeve to provide extra protection. The outer cover of almost all aircraft flexible hose is marked with a lay line, which consists of a yellow, red, or white stripe running the length of the hose. In addition to a stripe, the information needed to identify the hose, such as the MIL-SPEC number, the manufacturer’s name or symbol, the dash number representing the hose size, and in some cases, the manufacturer’s part number along with the year and quarter the hose was manufactured. In addition to identifying a hose, the lay line shows if a hose is twisted when it is installed. When a hose is installed properly, the lay line runs straight with no twists.
Flexible Line Identification Lay lines and identification markings consisting of lines, letters, and numbers are printed on the hose. Most hydraulic hose is marked to identify its type, the quarter and year of manufacture, and a 5digit code identifying the manufacturer. These markings are in contrasting coloured letters and numerals which indicate the natural lay (no twist) of the hose and are repeated at intervals of not more than 9 inches along the length of the hose. Code markings assist in replacing a hose with one of the same specifications or a recommended substitute. Hose suitable for use with phosphate ester base hydraulic fluid will be marked Skydrol use. In some instances, several types of hose may be suitable for the same use.
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Therefore, to make the correct hose selection, always refer to the applicable aircraft maintenance or parts manual.
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Size Designation The size of a flexible hose is determined by its inside diameter and is measured in increments of 1/16 inch. Like rigid tubing, a dash number indicates the tube diameter. For example, a -10 identifies a 10/16 or 5/8 inch hose.
Types Of Flexible Hose While aircraft hose is manufactured to meet a variety of applications, the types of hose are normally classified by the amount of pressure they are designed to withstand. These include lowpressure, medium-pressure, and high-pressure. Low-Pressure Hose Most air or vacuum hoses and some aircraft instrument lines are not required to carry high pressures. Therefore, low pressure rubber hose is typically used with these types of installations. These hoses have a seamless inner tube and a reinforcement made of a single layer of cotton braid. An outer cover of ribbed or smooth rubber is used to protect the reinforcement from physical abrasion.
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AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Medium-Pressure Hose Medium-pressure hose is used with fluid pressures up to 3,000 psi. However, its maximum operating pressure varies with its diameter. For example, smaller sizes carry pressure up to 3,000 psi while larger sizes are often restricted to lower pressures. Medium-pressure hose has a seamless inner liner with one layer of cotton braid and one layer of stainless-steel reinforcement. A braid of rough oil-resistant rubber-impregnated cotton is usually used as an outer cover. If the hose is used with a petroleum-based fluid, its inner liner is made of synthetic rubber and its outer braid is gray-black. However, if the hose is used with Skydrol or any phosphate-ester based hydraulic fluid, the inner liner is made of synthetic Butyl rubber and the outer braid is coloured green with SKYDROL written on it.
High-Pressure Hose All high-pressure hose has a maximum operating pressure of at least 3,000 psi and uses a synthetic rubber liner to carry petroleum products. This inner liner is wrapped with two or more steel braids as reinforcement. To help distinguish high-pressure hose from medium-pressure hose, the entire hose has a smooth outer cover. Most high-pressure hose is black with a yellow lay line. However, a hose designed to carry Skydrol has a Butyl rubber inner liner and a green outer cover with a white lay line.
Issue C: August 2009
Training Material Only Revision 1
B-6.6.1 and 6.6.2: Pipes and Unions Page 15 of 18
AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Fittings Fittings provide a convenient method of connecting flexible hoses to components. Flexible hose fittings are typically classified by the way they are attached to a hose and by the amount of pressure they can withstand.
Swaged End Fittings Hoses using swaged end fittings are assembled on special machinery that is typically not found in the shop. These fittings cannot be removed and reused. Therefore, replacement lines with swaged fittings must be obtained from the manufacturer or a properly equipped hose assembly shop.
Reusable Fittings Some hose assemblies incorporate reusable fittings consisting of a socket, a nipple, and a nut. When a failure occurs in a hose with this type of fitting, a replacement assembly can generally be fabricated. However, prior to reusing any fittings, they must be removed from the damaged hose assembly and care fully inspected. Any damage to the sealing surface or the threads is cause for rejection. Furthermore, the nut should be inspected for signs of cracking and damage caused by wrenches. Damaged components should not be reused. While all fittings used on aircraft must conform to MIL-SPEC, they are manufactured by several companies. Therefore, it is a good practice not to mix components manufactured by different companies.
To install a reusable fitting, begin by determining the length of the hose required. One way to do this is to use the damaged hose assembly with the fittings installed as a pattern. The length of hose required extends from the inside of one socket to the inside of the opposite socket.
Issue C: August 2009
Training Material Only Revision 1
B-6.6.1 and 6.6.2: Pipes and Unions Page 16 of 18
AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
One thing to keep in mind is that flexible fluid lines must have between five and eight percent slack to allow for the change in dimensions caused by fluid pressure. Under pressure, flexible hose contracts in length and expands in diameter.
Once you know the proper hose length, mark the cut length on an identical type of new hose and cut both ends off square. The cuts may be made with a cut ting tool designed especially for hose, or with a fine tooth hacksaw. After the hose is cut, place a socket in a vice taking care to protect the socket from the vice jaws. For low-pressure and some other types of fittings, a wooden vice block can be fabricated to hold the socket securely with minimal chance for damage. With the socket secured in the vice, screw the hose into the socket until it bottoms. Then, back off the hose just enough to prevent the rubber in the inner liner from obstructing the hole.
With the socket securely in place, lubricate the end of the assembly tool and force it into the hose to open the inner liner enough for the nipple to be inserted. The nipple screws into the socket with the assembly tool and squeezes the tube tight between the outside of the nipple and the inside of the socket. This squeezing action provides a strong physical attachment between the hose and the fitting, and forms a leak-proof seal.
Issue C: August 2009
Training Material Only Revision 1
B-6.6.1 and 6.6.2: Pipes and Unions Page 17 of 18
AA Form TO-18 B-6a Aircraft Materials and Corrosion
Part-66 Subject
Once the nipple is screwed completely into the socket, back it off from 1/32 to 1/16 inch to allow the nut to turn freely on the tube assembly. The inside of the nipple forms the sealing surface which mates with the flare cone of the fitting, and the nut pulls these two sealing surfaces together. When the assembly tool is screwed out of the hose, the inside of the hose should be blown out with compressed air and the entire hose inspected for physical condition.
Blanks When aircraft fluid lines are disconnected or removed it is a requirement that open ends of the lines and fittings are covered. Blanks are closed fittings, they can be made from either plastic, metal, wood or even a plastic bag. This prevents the ingress of contaminants into the system under maintenance. An appropriate cap plug should be fitted to disconnected lines or fittings and lightly torqued to prevent the fluid leakage, injury to personnel or damage to equipment if the open system is inadvertently energised.
Issue C: August 2009
Training Material Only Revision 1
B-6.6.1 and 6.6.2: Pipes and Unions Page 18 of 18